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.
Allow utility_navigation to be set to a unique component for rendering
Merge pull request #2164 from macfanatic/utility_navigation_setting

Allow utility_navigation to be set to a unique component for rendering
Merge pull request #2162 from jherdman/consolidate-engine-and-railtie

Remove Railtie, Use Rails Engine