Remove Railtie, Use Rails Engine

ActiveAdmin had both a Railtie and an Engine. This commit consolidates
this duplication to use engines, and moves locale files to their
conventional location for an Engine.

This commit is a step towards resolving #2126.
Merge pull request #2162 from jherdman/consolidate-engine-and-railtie

Remove Railtie, Use Rails Engine
only show empty_value for attr table if actually empty

Applying @jpmckinney's PR #1479 which never made it in.
Avoid NoMethodError for remember_me if it is not configured in devise