Shelling out was there for authors convenience, but we are
rather going to have the tag or SHA1 always in RAILS_VERSION
and if the environment variable is blank, then just use
"local" as a reminder that you are just working locally.

The docs server has been updated to set the long SHA1 in
RAILS_VERSION when generating edge guides.

The other way it was not marking the text as italic, it was showing the
underlines as normal text.

Also fixes some code examples indentation and # => marks in Active Model
and Active Record guides.

[ci skip]

The release notes is changed to include only the major changes and point
to the changelogs for the complete list. This lets us talk more about the
features included in the upcoming release and make the guide more useful
and interesting than just a list of all changelogs.

This is still a WIP guide pending a lot of work.

[ci skip]

It's really comfy being able to copy-paste the file paths.

I suppose there's also some benefit in trying to figure out the file name as a small exercise, but I don't think that sort of thing belongs in "Getting Started".

This introduces a bunch of editing/re-writing to the migrations guide.
There were a bunch of small changes, and a few larger ones:

* mysql does support transactions.
* Add a section about db/seeds.rb
* Largely re-wrote the first few sections

plus:

* 80 column formats
* replacing the explanation of the dynamic_form gem with the example
  HTML/CSS that Rails scaffolds generate.

Patched Marshal#load to work with constant autoloading (active_support/dependecies.rb) (issue #8167)

ActiveRecord::Observer was extracted into a separate gem so it
should not be referenced anymore (see https://github.com/rails/rails/commit/ccecab3)

Two big features that are only barely related in the same guide. Seems
bad.

I did not check references to these guides yet, so some links may need
to be updated.

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

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

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.

It's obvious that this is a guide.

Other guides that talk about Active Record specific features include the
name of the library in the title, this one should too for consistency.

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

Also, 'helpers' should be capitalized.

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