Register Comment as AdminComment

Fixes gregbell/active_admin#301.
Fix https://github.com/gregbell/active_admin/issues/1999: translation_missing error
Add a note regarding decorators in forms

[ci skip]
Merge pull request #2057 from carnesmedia/2013-document-decorator-form

Add a note regarding decorators in forms
Fix code sample indentation in arbre documentation
Fix false headings
Better heading hierarchy in arbre documentation
Add :allow_destroy option to has_many forms

This option will add a boolean field "Remove"
to the end of each has_many fieldset that is
an existing record. This provides a workaround
for issue #1990, and allows for cleaner form
coding.
Add i18n for has_many_remove label
Add OmniAuth provider links to the login page.
Merge pull request #2088 from henrrrik/2056-add-omniauth-provider-links-squashed

Add OmniAuth provider links to the login page.
IndexAsBlog now renders a proc passed to title/body methods in view context
Merge pull request #2083 from carnesmedia/indentation

fix Arbre documentation
Merge pull request #2087 from macfanatic/1890-blog-title-would-not-render-abre-components

IndexAsBlog now renders a proc passed to title/body methods in view context
Option to disable automatic decoration in forms

Ref: #2057
Resolves #1670 by letting scope DSL accept a proc for lazy evaluation
Merge pull request #2085 from carnesmedia/forms-without-decorators

Add option to "undecorate" resource for forms
Merge pull request #2084 from macfanatic/default-scope-accept-a-proc

#1670 - :default now expects proc for index scopes
fix breadcrumb links for namespaced parent models

The previous code broke nested resource (e.g. /users/1/stories) had a
parent class that was namespaced inside a module.

```ruby
# Given these models
class Foo::Users   # has_many stories
class Foo::Stories # belongs_to user

# and this AA routing for stories
ActiveAdmin.register Foo::Stories do
  belongs_to :user, parent_class: Foo::User
end

# you'd get this error
"undefined method `find_by_id' for Foo:Module"
```

This commit resolves the problem by directly asking the AA config
what the parent class is, instead of trying to guess based on the URL.

Fixes #2051
RSpec has been standard issue for a while, so `should` is a given