Add Performance/StringIdentifierArgument rule

It was added in rubocop-performance 1.13.0. We were already applying it
in most places.

We aren't adding it for performance reasons but in order to make the
code more consistent.

spec/shared/system/relationable.rb

@@ -173,11 +173,11 @@ shared_examples "relationable" do |relationable_model_name|
     related_content = create(:related_content, parent_relationable: relationable, child_relationable: related1, author: build(:user))
 
     2.times do
-      related_content.send("score_positive", build(:user))
+      related_content.send(:score_positive, build(:user))
     end
 
     6.times do
-      related_content.send("score_negative", build(:user))
+      related_content.send(:score_negative, build(:user))
     end
 
     login_as(user)