Upgrade to Rails 5.2

All the code in the `bin/` and the `config/` folder has been generated
running `rake app:update`, except the `escape_javascript_fix` file,
which we've removed since the code there is already included in Rails
5.2.

.rubocop.yml

@@ -253,7 +253,6 @@ Rails/OutputSafety:
   Severity: warning
   Exclude:
     - app/helpers/text_with_links_helper.rb
-    - config/initializers/escape_javascript_fix.rb
 
 Rails/PluralizationGrammar:
   Enabled: true