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
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
hook filters into Devise controllers just like resource controllers

kudos to @JangoSteve for identifying this and doing the initial
implementation

Closes #2183
Fixing-documentation-typo-related-to-cancan
remove needless Formtastic overrides

This has the added benefit of resolving #2170
Merge pull request #2188 from prdetective/2187-fixing-documentation-typo-related-to-cancan

Fix documentation typo
Some ORMs/ODMs return a Symbol for primary key, therefore we must call #to_s (or interpolate as was the case before @Daxter's 5a736047)
Do not assume that a #foo_id method implies a #foo method (fixes @Daxter's regression in 600b5398)
starting to integrate changes for #2002
adding the changes for locales #2002
updating test #2002
fixing the dropdown issue #2002
moving the validation to the correct method in forms.rb #2002
testing a no change commit #2002