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/budget_spec.rb

@@ -22,7 +22,7 @@ describe Budget do
     end
 
     it "may be repeated for the same budget and a different locale" do
-      budget.update(name_fr: "object name")
+      budget.update!(name_fr: "object name")
       expect(budget.translations.last).to be_valid
     end
   end
@@ -45,7 +45,7 @@ describe Budget do
       before do
         budget.phases.each do |phase|
           phase.description = phase.kind.humanize
-          phase.save
+          phase.save!
         end
       end
 
@@ -273,28 +273,28 @@ describe Budget do
 
   describe "#formatted_amount" do
     it "correctly formats Euros with Spanish" do
-      budget.update(currency_symbol: "€")
+      budget.update!(currency_symbol: "€")
       I18n.locale = :es
 
       expect(budget.formatted_amount(1000.00)).to eq "1.000 €"
     end
 
     it "correctly formats Dollars with Spanish" do
-      budget.update(currency_symbol: "$")
+      budget.update!(currency_symbol: "$")
       I18n.locale = :es
 
       expect(budget.formatted_amount(1000.00)).to eq "1.000 $"
     end
 
     it "correctly formats Dollars with English" do
-      budget.update(currency_symbol: "$")
+      budget.update!(currency_symbol: "$")
       I18n.locale = :en
 
       expect(budget.formatted_amount(1000.00)).to eq "$1,000"
     end
 
     it "correctly formats Euros with English" do
-      budget.update(currency_symbol: "€")
+      budget.update!(currency_symbol: "€")
       I18n.locale = :en
 
       expect(budget.formatted_amount(1000.00)).to eq "€1,000"
@@ -314,7 +314,7 @@ describe Budget do
 
     it "returns array of investments milestone_tags" do
       investment1.milestone_tag_list = "tag1"
-      investment1.save
+      investment1.save!
       budget.investments << investment1
 
       expect(budget.milestone_tags).to eq(["tag1"])
@@ -322,9 +322,9 @@ describe Budget do
 
     it "returns uniq list of investments milestone_tags" do
       investment1.milestone_tag_list = "tag1"
-      investment1.save
+      investment1.save!
       investment2.milestone_tag_list = "tag1"
-      investment2.save
+      investment2.save!
       budget.investments << investment1
       budget.investments << investment2
 
@@ -333,9 +333,9 @@ describe Budget do
 
     it "returns tags only for winner investments" do
       investment1.milestone_tag_list = "tag1"
-      investment1.save
+      investment1.save!
       investment3.milestone_tag_list = "tag2"
-      investment3.save
+      investment3.save!
       budget.investments << investment1
       budget.investments << investment3