Add support for specifying non-default operator classes in PostgreSQL
indexes. An example CREATE INDEX query that becomes possible is:

    CREATE INDEX users_name ON users USING gist (name gist_trgm_ops);

Previously it was possible to specify the `gist` index but not the
custom operator class. The `add_index` call for the above query is:

    add_index :users, :name, using: :gist, opclasses: {name: :gist_trgm_ops}

and abstract `SchemaDumper#extensions` is now an empty method.

Since #30337, every database adapter has its own `SchemaDumper`.
`extensions` are only supported by PostgreSQL database and postgresql database adapter.

I was added a table options after `force: :cascade` in #17569 for not
touching existing tests (reducing diff). But `force: :cascade` is not an
important information. So I prefer to place a table options before
`force: :cascade`.

Currently `SchemaDumper` is only customizable for column options. But
3rd party connection adapters (oracle-enhanced etc) need to customizable
for table or index dumping also. To make it possible, I introduced
adapter specific `SchemaDumper` classes for that.

MySQL 5.7 and PostgreSQL 9.6 allow table identifiers have the dollar sign.

* MySQL 5.7
https://dev.mysql.com/doc/refman/5.7/en/identifiers.html

> Permitted characters in unquoted identifiers:
>  ASCII: [0-9,a-z,A-Z$_] (basic Latin letters, digits 0-9, dollar, underscore)

* PostgreSQL 9.6
https://www.postgresql.org/docs/9.6/static/sql-syntax-lexical.html

> SQL identifiers and key words must begin with a letter (a-z, but also letters with diacritical marks and non-Latin letters) or an underscore (_). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), or dollar signs ($). Note that dollar signs are not allowed in identifiers according to the letter of the SQL standard, so their use might render applications less portable. The SQL standard will not define a key word that contains digits or starts or ends with an underscore, so identifiers of this form are safe against possible conflict with future extensions of the standard.

Address #30044

[Yasuo Honda & Ryuta Kamizono]

The list of enabled adapter extensions in the schema dump isn't
sorted by default, so it may happen that the sorting changes
over time. If you're using a VCS, a change to the sorting results
in a diff without any real change. Sorting the list should solve
this problem.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

* Fix the comment on `formatted_version`
* Extract `define_params`
* Remove duplicated guard clause for `@version`

`tables` returns only tables now.

Add `default_index_type?` to the generic schema dumper doesn't have the knowledge about an index type

Follow up to #26735.

If `table_options` returns `{ comment: nil }`, `create_table` line is
broken.

Example:

```ruby
  create_table "accounts", force: :cascade,  do |t|
```

`migration_keys` includes `name` but `name` is not a column option.

```ruby
  # Before
  t.index ["firm_id", "type", "rating"], name: "company_index", order: {"rating"=>:desc}, using: :btree

  # After
  t.index ["firm_id", "type", "rating"], name: "company_index", order: { rating: :desc }, using: :btree
```

Currently blank comment does not dump to `db/schema.rb`. But created it
even if specified blank.

`format_string` is used for standardized column types/arguments spaces.
Now the standardization was removed at df84e986.

Now the schema dumper by default doesn't align the types and arguments
in the ruby format anymore.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Actually schema dumper/creation supports composite primary key (#21614).
Therefore it should not show the warning about composite primary key in
connection adapter.

This change moves the warning to `AttributeMethods::PrimaryKey` and
suppress the warning for habtm join table.

Fixes #25388.

Rails 5.0 has been dropped Ruby 1.9 support.
I think no need magic comment anymore.

If the adapter supports indexes in create table, it generates SQL
in one query.

* Switch to keyword args where we can without breaking compat.
* Use add_table_options! for :options, too.
* Some code polish.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

Currently the results of `column.serial?` is not correct. For
`column.serial?` correctly working, initialize `column.table_name`
immediately.

This reverts commit 99801c6a.

Ultimately it doesn't matter whether `add_index` or `t.index` are used
in the schema dumper in any meaningful way. There are gems out there
which hook into the old behavior for things like indexing materialized
views. Since the reverted commit doesn't seem to add much benefit,
there's no reason for us to break these gems.

This PR introduces a key/value type store to Active Record that can be used for storing internal values. It is an alternative implementation to #21237 cc @sgrif @matthewd.

It is possible to run your tests against your production database by accident right now. While infrequently, but as an anecdotal data point, Heroku receives a non-trivial number of requests for a database restore due to this happening. In these cases the loss can be large.

To prevent against running tests against production we can store the "environment" version that was used when migrating the database in a new internal table. Before executing tests we can see if the database is a listed in `protected_environments` and abort. There is a manual escape valve to force this check from happening with environment variable `DISABLE_DATABASE_ENVIRONMENT_CHECK=1`.

These new methods are used from the Active Record model layer to
determine which relations are viable to back a model. These new methods
allow us to change `conn.tables` in the future to only return tables and
no views. Same for `conn.table_exists?`.

The goal is to provide the following introspection methods on the
connection:

* `tables`
* `table_exists?`
* `views`
* `view_exists?`
* `data_sources` (views + tables)
* `data_source_exists?` (views + tables)