Apply Layout/LineLength rubocop rule

Note we're excluding a few files:

* Configuration files that weren't generated by us
* Migration files that weren't generated by us
* The Gemfile, since it includes an important comment that must be on
  the same line as the gem declaration
* The Budget::Stats class, since the heading statistics are a mess and
  having shorter lines would require a lot of refactoring

spec/controllers/moderation/users_controller_spec.rb

@@ -30,7 +30,8 @@ describe Moderation::UsersController do
       put :block, params: { id: user }
 
       expect(response).to redirect_to proposals_path
-      expect(flash[:notice]).to eq "The user has been blocked. All contents authored by this user have been hidden."
+      expect(flash[:notice]).to eq "The user has been blocked. " \
+                                   "All contents authored by this user have been hidden."
     end
 
     it "redirects to the index with a nested resource" do