Only use association reflections for filters

Edge Case:

Model.reflections typically only returns association reflections.
However, there is a case where it may return other reflections. For
instance, if a model uses the `compose_of` macro, the attribute for
which `composed_of` is called shows up as a reflection, despite not
being an association.

This makes sure that #default_association_filters will only return
association filters. It also cleans up the code a bit since we end
up working with an Array instead of a Hash.
Add ORM-specific loaders:
- lib\active_admin\orm\active_record
- lib\active_admin\orm\mongoid

Move comments to ActiveRecord ORM directory

Add ActiveRecord/Mongoid type checks to DisplayHelper#pretty_format(object) method
update the show page docs
DisplayHelper#pretty_format fix indentation, remove nested if/else from case statement
Merge pull request #2545 from johnnyshields/orm-agnosticism

Make ActiveAdmin ORM agnostic (i.e. works without loading ActiveRecord)
Merge pull request #2539 from shekibobo/association-filter-fixup

Only use association reflections for filters
Support select filters for has_many :through
Support check boxes filters for has_many :through
Merge pull request #2541 from shekibobo/filters-has-many-through

Automatically build has_many :through filters.
update CONTRIBUTING.md
update database_cleaner

closes #2388
only show the "New" action item on index pages

closes #894
code style update
redirect_to :back on comment creation

fixes #2269
update the list of dependencies
fix #1364 by not duplicating the default table presenter

It turns out we had the same default block in two places, but one was
out of date. Now we just rely on IndexAsTable.
fix typo in docs
#2537: autoload the comments model

By not loading the Comments model until Rails is initialized, we can
rely on the user's changes to Kaminari settings.
update the CHANGELOG
Fix translations for attributes_table component when using `row`

For some reason `@resource` from `AttributeTable` is `nil`, only
`@collection` is available. This fixes try to get the `@resource` class
from both `@resource` and `@collection`

**Issue:** #2556