Getting Started: remove reference to attr_accessible
Getting Started: Improve instructions on opening a terminal
Migrations: move massive paragraph out of intro.

Most of the guides have a few simple sentences describing what they will show
you at the top. This one had a few big paragraphs. I've moved those paragraphs
down to an introductory one, and written a new smaller one for the introduction.
This makes this guide more consistent with the others.
ActiveRecord -> Active Record

The proper name, as per the documentation guides, is 'Active Record.' I've
updated several references to it as 'ActiveRecord'.
ActionPack -> Action Pack.

The proper name has a space.
ActionMailer -> Action Mailer.

The proper name for the library has a space.
ActiveSupport -> Active Support

The proper name has a space.
ActionController -> Action Controller

The proper name has a space.
ActionView -> Action View

The proper name has a space.
Migrations: Fix opening bullets.

The wording of these was a bit off, so I fixed them.
Add periods to the bullet points in guides.

Talked with @fxn about this. Bullet points should have periods at the ends.
Remove 'This guide is based on ' warnings.

It's obvious that the guide is based on this version of Rails, and may not be backwards compatible.
Associations: s/A Guide//

It's obvious that this is a guide, and none of the other guides say 'A Guide to'.
Fix title of 'form helpers' guide.

They're Rails Guides, so obviously we're talking about Rails Form Helpers.

Also, 'helpers' should be capitalized.
Migrations: add 'Active Record' to title

Other guides that talk about Active Record specific features include the
name of the library in the title, this one should too for consistency.
Command Line: remove 'A Guide to'

It's obvious that this is a guide.
Asset Pipeline: add 'the'

Some guides work without 'the'. For instance, 'Migrations' or
'Form Helpers.' But when we talk about the asset pipeline, we...
always say 'the asset pipeline.' The guide title should reflect this.
Move instantiation responsibilities from Inheritance to Persistence. Have Inheritance#discriminate_class_for_record handle STI lookup duties.
Relation.where with no args can be chained with not, like, and not_like

examples:

  Model.where.not field: nil
  #=> "SELECT * FROM models WHERE field IS NOT NULL

  Model.where.like name: 'Jeremy%'
  #=> "SELECT * FROM models WHERE name LIKE 'Jeremy%'

this feature was originally suggested by Jeremy Kemper https://github.com/rails/rails/pull/5950#issuecomment-5591330

Closes #5950
Gemfile source url for plugins updated