Fix deprecation warnings in Rails 5.1

[shioyama]

There are tons of deprecation warnings currently in Rails/AR 5.1. This removes the warnings while still passing specs. However, it introduces some questionable changes to achieve this.

There are two problems:

1. AR::Dirty now requires you to declare all attributes upfront rather than in an open-ended way with attr_will_change! (see Check for forced attribute changes rails/rails#27963). To get around this, I've overridden has_attribute? to make locale accessors (title_en etc) act like real attributes. We can't actually define them all since the list of locales is open-ended.
2. You can't call changes from within after save callbacks. This conflicts with code used to support previous_changes in the Mobility Dirty backend module. This is fixed using a before_save hook rather than a changes_applied override, but this may fail to work as expected if the model calls previous_changes in another before_save callback added after this.

The first solution is ok I think for now, the second one may be more problematic but at least is limited to previous_changes only.

[shioyama]

For the record, I got around the second issue by overriding changes_internally_applied in AR >= 5.1.