remove link to edgeapi [ci skip]

Improve linking inside guides

To ease reading the "Active Support Core Extensions" guide, let's
automatically link references to Ruby files.

It's also possible to reference other components' files in the even
though it's not used in the guides at the moment.

[Petr Skocik & Robin Dupret]

To make sure that the user won't look at a feature that doesn't
already exist if they are looking at a previous version of the guides,
let's automatically inject the Rails version the guides have been
generated against.

Deprecate locking of dirty records

Remove deprecated passing string to define callback

Revert "Remove useless `column_alias` in `subquery_for_count`"

This reverts commit 28977f1f.

`primary_key` and `references` columns should be identical type

Move some ActionCable logs to debug level

Decouple the building Arel ASTs for uniqueness validator

If select clause is specified and last column has a column alias,
additional column alias causes a statement invalid.

Add test coverage for counting a single column with NULL values.

Fixes #27676, #27682, and #27705.

Follow up to #26266.

The default type of `primary_key` and `references` were changed to
`bigint` since #26266. But legacy migration and sqlite3 adapter should
keep its previous behavior.

Restore the behaviour of the compatibility layer for integer-like PKs

* kamipo/fix_mysql_pk_dumping_correctly:
  Restore custom primary key tests lost at #26266
  Restore the behaviour of the compatibility layer for integer-like PKs
  Correctly dump integer-like primary key with default nil

Deprecate `ColumnDumper#migration_keys`

Remove superfluous `MySQL::Column` tests

`ColumnDumper#migration_keys` was extracted to customize keys for
standardized column arguments widths. But the feature was removed in
df84e986. The internal method is no longer used for that.

Since text default treated as an empty string in non-strict mode has
been removed in #26154, `MySQL::Column` behaves like any other column.
Also, The difference between strict and non-strict mode is covered by
`test_mysql_not_null_defaults_strict` and `test_mysql_not_null_defaults_non_strict`.

Remove `ActiveModel::TestCase` from lib

`ActiveModel::TestCase` is used only for the test of Active Model.
Also, it is a private API and can not be used in applications.
Therefore, it is not necessary to include it in lib.

Correct spelling

Add test for DateTime#blank?

Indicate action that failed in YamlColumn

```
go get -u github.com/client9/misspell/cmd/misspell
misspell  -w -error -source=text .
```

Add `:args` to `process.action_mailer` event.

Collect all file patterns when running multiple rake test tasks

Docs: Correction: Module::DelegationError

Replaces the rake_patterns instance variable with simple require, as
`autorun` will run tests from all eagerly required test files.

Fixes #27801

Some custom primary key tests (added at #17631, #17696, #18220, #18228)
were lost at #26266. Restore the tests to improve test coverage.

The PR #27384 changed migration compatibility behaviour.

```ruby
class CreateMasterData < ActiveRecord::Migration[5.0]
  def change
    create_table :master_data, id: :integer do |t|
      t.string :name
    end
  end
end

```

Previously this migration created non-autoincremental primary key
expected. But after the PR, the primary key changed to autoincremental,
it is unexpected.

This change restores the behaviour of the compatibility layer.