Create a separate ActiveRecord locale

Decouples pluralization activities from the current locale.

https://github.com/rails/rails/pull/10158 changes pluralization to depend on the current locale, which breaks ActiveRecord which makes heavy use of pluralization and depends on this function returning consistent results.  You can see the results http://intertwingly.net/projects/AWDwR4/checkdepot/section-15.2.html where we get a `Could not find table 'cart'` error message when we switch locales.

Preferred solutions are to either revert the original commit, or to give ActiveRecord a separate locale.  This pull request implements the latter.  I'm OK with either option.