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

app/controllers/concerns/admin/budget_headings_actions.rb

@@ -59,7 +59,8 @@ module Admin::BudgetHeadingsActions
     end
 
     def allowed_params
-      valid_attributes = [:price, :population, :allow_custom_content, :latitude, :longitude, :max_ballot_lines, :geozone_id]
+      valid_attributes = [:price, :population, :allow_custom_content, :latitude, :longitude,
+                          :max_ballot_lines, :geozone_id]
 
       [*valid_attributes, translation_params(Budget::Heading)]
     end