Make `db:migrate:status` to render `1_some.rb` format migrate files.

Fix test_database_created_by_root of mysql

`DEFAULT_CHARSET` and `DEFAULT_COLLATION` in `MySQLDatabaseTasks`
was changed by 322068fe.
This test case also should be changed.

[ci skip] querying guide methods first and last: mentioning the influence of the default scope

`1_valid_people_have_last_names.rb` and
`20150823202140_create_users.rb` are valid migration file name.
But `1_valid_people_have_last_names.rb` is rendered as
`********** NO FILE **********` when `rake db:migrate:status`.

Fix to this bug, this commit includes

* define some API private methdos and a Constant
  `match_to_migration_filename?`, `parse_migration_filename`, and
  `MigrationFilenameRegexp`
* use these methods in `db:migrate:status` task

Example:

These files are in `db/migrate`

* 1_valid_people_have_last_names.rb
* 20150819202140_irreversible_migration.rb
* 20150823202140_add_admin_flag_to_users.rb
* 20150823202141_migration_tests.rb
* 2_we_need_reminders.rb
* 3_innocent_jointable.rb

we can migrate all of them.

Before

```shell
$ bundle exec rake db:migrate:status

...

 Status   Migration ID    Migration Name
--------------------------------------------------
   up     001             ********** NO FILE **********
   up     002             ********** NO FILE **********
   up     003             ********** NO FILE **********
   up     20150819202140  Irreversible migration
   up     20150823202140  Add admin flag to users
   up     20150823202141  Migration tests
```

After

```shell
$ bundle exec rake db:migrate:status

...

 Status   Migration ID    Migration Name
--------------------------------------------------
   up     001             Valid people have last names
   up     002             We need reminders
   up     003             Innocent jointable
   up     20150819202140  Irreversible migration
   up     20150823202140  Add admin flag to users
   up     20150823202141  Migration tests
```

remove unnecessary `.sass-cache` from plugin's gitignore template

Define `sanitize_sql_for_order` for AR and use it inside `preprocess_…

This commit follows up of 6a6dbb4c.

Allow bigint with default nil for avoiding auto increment primary key

Such as #10404, #18206.

Remove `DEFAULT_CHARSET` and `DEFAULT_COLLATION`

This reverts commit f6ca7e4e.

The default collation of utf8 in MySQL is the `utf8_general_ci`, and
this should not be changed. This is because, the better collation in the
all locales is not exists, optimal collation in own application is not
known other than themselves.

The `utf8_unicode_ci` is known as Japanese killer in Japan, there are
serious impacts in search of Japanese.

MySQL implements the `utf8_unicode_ci` according to the Unicode
Collation Algorithm (UCA) described at http://www.unicode.org/reports/tr10/,
but the `utf8_unicode_ci` have only partial support for the UCA, only
primary level key comparison implemented (also known as L1 (Base
characters) comparison).

Because L1 (Base characters) comparison does not distinguish between the
presence or absence of the accent, if distinction of the accent is
important there is a serious impact (e.g. Japanese).

Example:

```
> SHOW CREATE TABLE `dicts`\G
*************************** 1. row ***************************
       Table: dicts
Create Table: CREATE TABLE `dicts` (
  `word` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `meaning` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
1 row in set (0.00 sec)

> INSERT INTO `dicts` VALUES ('ハハ', 'mother'), ('パパ', 'father');
Query OK, 2 rows affected (0.00 sec)

> SELECT * FROM `dicts` WHERE `word` = 'ハハ';
+--------+---------+
| word   | meaning |
+--------+---------+
| ハハ   | mother  |
| パパ   | father  |
+--------+---------+
2 rows in set (0.00 sec)

> CREATE UNIQUE INDEX `unique_index_word` ON `dicts`(`word`);
ERROR 1062 (23000): Duplicate entry 'ハハ' for key 'unique_index_word'
```

We should omit the collation entirely rather than providing a default.
Then the choice is the responsibility of the server and MySQL distribution.

Add CHANGELOG entry for #22125 [ci skip]

Move some AR test cases to inheritance_test.rb

Since the sass cache is output to the `tmp/cache/sass`.

Update changelog for #22122

Fix double word 'string' [ci skip]

Route generator should be idempotent

Fix double word 'be' [ci skip]

minor formatting changes [ci skip]

* add newline for display the fenced code block
* add "#" in the comments section

These methods are defined in inheritance.rb

* `abstract_class?`
* `descends_from_active_record?`
* `compute_type`

this allows us to construct mailer objects without possibly disastrous
side-effects.

Alias left_joins to left_outer_joins

Add test for parsing application/vnd.api+json

rails/application: allow passing an env to config_for

we should be using a countdown latch instead of rolling our own
busy-loop.

Make migrations concurrent safe (using advisory locks)