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/documents_controller_spec.rb

@@ -10,7 +10,8 @@ describe DocumentsController do
         document = create(:document, documentable: current_answer)
         delete :destroy, params: { id: document }
 
-        expect(flash[:alert]).to eq "You do not have permission to carry out the action 'destroy' on Document."
+        expect(flash[:alert]).to eq "You do not have permission to " \
+                                    "carry out the action 'destroy' on Document."
         expect(Document.count).to eq 1
       end