Use Rails 5 conventions in migrations and models

We forgot to add these changes to pull requests which were in
development before we upgraded to Rails 5.

We're also moving the rubocop rules to the basic files, so we're
notified when we inherit from `ActiveRecord::Base`.

.rubocop.yml

@@ -93,18 +93,9 @@ Performance/UnfreezeString:
 Performance/UriDefaultParser:
   Enabled: true
 
-Rails/ActionFilter:
-  Enabled: true
-
 Rails/ActiveSupportAliases:
   Enabled: true
 
-Rails/ApplicationJob:
-  Enabled: true
-
-Rails/ApplicationRecord:
-  Enabled: true
-
 Rails/Blank:
   Enabled: true