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/debate_spec.rb

@@ -133,19 +133,19 @@ describe Debate do
     end
 
     it "is true for anonymous users if allowed anonymous votes" do
-      debate.update(cached_anonymous_votes_total: 420, cached_votes_total: 1000)
+      debate.update!(cached_anonymous_votes_total: 420, cached_votes_total: 1000)
       user = create(:user)
       expect(debate.votable_by?(user)).to be true
     end
 
     it "is true for anonymous users if less than 100 votes" do
-      debate.update(cached_anonymous_votes_total: 90, cached_votes_total: 92)
+      debate.update!(cached_anonymous_votes_total: 90, cached_votes_total: 92)
       user = create(:user)
       expect(debate.votable_by?(user)).to be true
     end
 
     it "is false for anonymous users if too many anonymous votes" do
-      debate.update(cached_anonymous_votes_total: 520, cached_votes_total: 1000)
+      debate.update!(cached_anonymous_votes_total: 520, cached_votes_total: 1000)
       user = create(:user)
       expect(debate.votable_by?(user)).to be false
     end
@@ -417,44 +417,44 @@ describe Debate do
   describe "conflictive debates" do
 
     it "returns true when it has more than 1 flag for 5 positive votes" do
-      debate.update(cached_votes_up: 4)
-      debate.update(flags_count: 1)
+      debate.update!(cached_votes_up: 4)
+      debate.update!(flags_count: 1)
       expect(debate).to be_conflictive
 
-      debate.update(cached_votes_up: 9)
-      debate.update(flags_count: 2)
+      debate.update!(cached_votes_up: 9)
+      debate.update!(flags_count: 2)
       expect(debate).to be_conflictive
 
-      debate.update(cached_votes_up: 14)
-      debate.update(flags_count: 3)
+      debate.update!(cached_votes_up: 14)
+      debate.update!(flags_count: 3)
       expect(debate).to be_conflictive
 
-      debate.update(cached_votes_up: 2)
-      debate.update(flags_count: 20)
+      debate.update!(cached_votes_up: 2)
+      debate.update!(flags_count: 20)
       expect(debate).to be_conflictive
     end
 
     it "returns false when it has less than or equal to 1 flag for 5 positive votes" do
-      debate.update(cached_votes_up: 5)
-      debate.update(flags_count: 1)
+      debate.update!(cached_votes_up: 5)
+      debate.update!(flags_count: 1)
       expect(debate).not_to be_conflictive
 
-      debate.update(cached_votes_up: 10)
-      debate.update(flags_count: 2)
+      debate.update!(cached_votes_up: 10)
+      debate.update!(flags_count: 2)
       expect(debate).not_to be_conflictive
 
-      debate.update(cached_votes_up: 100)
-      debate.update(flags_count: 2)
+      debate.update!(cached_votes_up: 100)
+      debate.update!(flags_count: 2)
       expect(debate).not_to be_conflictive
     end
 
     it "returns false when it has no flags" do
-      debate.update(flags_count: 0)
+      debate.update!(flags_count: 0)
       expect(debate).not_to be_conflictive
     end
 
     it "returns false when it has not votes up" do
-      debate.update(cached_votes_up: 0)
+      debate.update!(cached_votes_up: 0)
       expect(debate).not_to be_conflictive
     end