Apply Rails/SaveBang rubocop rule

Having exceptions is better than having silent bugs.

There are a few methods I've kept the same way they were.

The `RelatedContentScore#score_with_opposite` method is a bit peculiar:
it creates scores for both itself and the opposite related content,
which means the opposite related content will try to create the same
scores as well.

We've already got a test to check `Budget::Ballot#add_investment` when
creating a line fails ("Edge case voting a non-elegible investment").

Finally, the method `User#send_oauth_confirmation_instructions` doesn't
update the record when the email address isn't already present, leading
to the test "Try to register with the email of an already existing user,
when an unconfirmed email was provided by oauth" fo fail if we raise an
exception for an invalid user. That's because updating a user's email
doesn't update the database automatically, but instead a confirmation
email is sent.

There are also a few false positives for classes which don't have bang
methods (like the GraphQL classes) or destroying attachments.

For these reasons, I'm adding the rule with a "Refactor" severity,
meaning it's a rule we can break if necessary.

spec/models/setting_spec.rb

@@ -19,64 +19,64 @@ describe Setting do
 
   describe "#prefix" do
     it "returns the prefix of its key" do
-      expect(Setting.create(key: "prefix.key_name").prefix).to eq "prefix"
+      expect(Setting.create!(key: "prefix.key_name").prefix).to eq "prefix"
     end
 
     it "returns the whole key for a non prefixed key" do
-      expect(Setting.create(key: "key_name").prefix).to eq "key_name"
+      expect(Setting.create!(key: "key_name").prefix).to eq "key_name"
     end
   end
 
   describe "#type" do
     it "returns the key prefix for 'process' settings" do
-      process_setting = Setting.create(key: "process.whatever")
+      process_setting = Setting.create!(key: "process.whatever")
       expect(process_setting.type).to eq "process"
     end
 
     it "returns the key prefix for 'feature' settings" do
-      feature_setting = Setting.create(key: "feature.whatever")
+      feature_setting = Setting.create!(key: "feature.whatever")
       expect(feature_setting.type).to eq "feature"
     end
 
     it "returns the key prefix for 'map' settings" do
-      map_setting = Setting.create(key: "map.whatever")
+      map_setting = Setting.create!(key: "map.whatever")
       expect(map_setting.type).to eq "map"
     end
 
     it "returns the key prefix for 'html' settings" do
-      html_setting = Setting.create(key: "html.whatever")
+      html_setting = Setting.create!(key: "html.whatever")
       expect(html_setting.type).to eq "html"
     end
 
     it "returns the key prefix for 'homepage' settings" do
-      homepage_setting = Setting.create(key: "homepage.whatever")
+      homepage_setting = Setting.create!(key: "homepage.whatever")
       expect(homepage_setting.type).to eq "homepage"
     end
 
     it "returns the key prefix for 'remote_census.general' settings" do
-      remote_census_general_setting = Setting.create(key: "remote_census.general.whatever")
+      remote_census_general_setting = Setting.create!(key: "remote_census.general.whatever")
       expect(remote_census_general_setting.type).to eq "remote_census.general"
     end
 
     it "returns the key prefix for 'remote_census_request' settings" do
-      remote_census_request_setting = Setting.create(key: "remote_census.request.whatever")
+      remote_census_request_setting = Setting.create!(key: "remote_census.request.whatever")
       expect(remote_census_request_setting.type).to eq "remote_census.request"
     end
 
     it "returns the key prefix for 'remote_census_response' settings" do
-      remote_census_response_setting = Setting.create(key: "remote_census.response.whatever")
+      remote_census_response_setting = Setting.create!(key: "remote_census.response.whatever")
       expect(remote_census_response_setting.type).to eq "remote_census.response"
     end
 
     it "returns 'configuration' for the rest of the settings" do
-      configuration_setting = Setting.create(key: "whatever")
+      configuration_setting = Setting.create!(key: "whatever")
       expect(configuration_setting.type).to eq "configuration"
     end
   end
 
   describe "#enabled?" do
     it "is true if value is present" do
-      setting = Setting.create(key: "feature.whatever", value: 1)
+      setting = Setting.create!(key: "feature.whatever", value: 1)
       expect(setting.enabled?).to eq true
 
       setting.value = "true"
@@ -87,7 +87,7 @@ describe Setting do
     end
 
     it "is false if value is blank" do
-      setting = Setting.create(key: "feature.whatever")
+      setting = Setting.create!(key: "feature.whatever")
       expect(setting.enabled?).to eq false
 
       setting.value = ""
@@ -97,18 +97,18 @@ describe Setting do
 
   describe "#content_type?" do
     it "returns true if the last part of the key is content_types" do
-      expect(Setting.create(key: "key_name.content_types").content_type?).to be true
+      expect(Setting.create!(key: "key_name.content_types").content_type?).to be true
     end
 
     it "returns false if the last part of the key is not content_types" do
-      expect(Setting.create(key: "key_name.whatever").content_type?).to be false
+      expect(Setting.create!(key: "key_name.whatever").content_type?).to be false
     end
   end
 
   describe "#content_type_group" do
     it "returns the group for content_types settings" do
-      images =    Setting.create(key: "update.images.content_types")
-      documents = Setting.create(key: "update.documents.content_types")
+      images =    Setting.create!(key: "update.images.content_types")
+      documents = Setting.create!(key: "update.documents.content_types")
 
       expect(images.content_type_group).to    eq "images"
       expect(documents.content_type_group).to eq "documents"