Fixes interpolation on SafeBuffer

Interpolation was untested and did not work with hash arguments.

Adds
- support for interpolation with hash argument
- tests for the above
- tests for safe/unsafe interpolation
Fix url leak in application templates guide

Encapsulate url that was including the trailing quote and colon.
Remove the link for code.whytheluckystiff.net

This is not a valid URL.

[ci skip]
Better support for `where()` conditions that use an association name.

Using the name of an association in `where` previously worked only
if the value was a single `ActiveRecrd::Base` object. e.g.

    Post.where(author: Author.first)

Any other values, including `nil`, would cause invalid SQL to be
generated. This change supports arguments in the `where` query
conditions where the key is a `belongs_to` association name and the
value is `nil`, an `Array` of `ActiveRecord::Base` objects, or an
`ActiveRecord::Relation` object.

    # Given the Post model
    class Post < ActiveRecord::Base
      belongs_to :author
    end

    # nil value finds records where the association is not set
    Post.where(author: nil)
    # SELECT "posts".* FROM "posts" WHERE "posts"."author_id" IS NULL

    # Array values find records where the association foreign key
    # matches the ids of the passed ActiveRecord models, resulting
    # in the same query as Post.where(author_id: [1,2])
    authors_array = [Author.find(1), Author.find(2)]
    Post.where(author: authors_array)

    # ActiveRecord::Relation values find records using the same
    # query as Post.where(author_id: Author.where(last_name: "Emde"))
    Post.where(author: Author.where(last_name: "Emde"))

Polymorphic `belongs_to` associations will continue to be handled
appropriately, with the polymorphic `association_type` field added
to the query to match the base class of the value. This feature
previously only worked when the value was a single `ActveRecord::Base`.

    class Post < ActiveRecord::Base
      belongs_to :author, polymorphic: true
    end

    Post.where(author: Author.where(last_name: "Emde"))
    # Generates a query similar to:
    Post.where(author_id: Author.where(last_name: "Emde"), author_type: "Author")
Fix method redefinition warning
Add an application controller for internal controllers
Add mailer previews feature based on mail_view gem
<tt> mismatches [ci skip]
Extra trailing period [ci skip]
Enum example in the release notes is wrong [ci skip]
Highlight before_action [ci skip]
Removed empty documentation section [ci skip]
Enum guides URL is already outdated [ci skip]
Update 4.1 highlights [ci skip]
Clearified what Spring actually does [ci skip]
Variants is probably too generic? [ci skip]
Added a section for mailer previews in the release notes [ci skip]
Re-arranged things to match the highlights at the top [ci skip]
Put Spring before the code-level features to group things a little bit better [ci skip]
Upgrading from what? [ci skip]