Merge remote-tracking branch 'EXreaction/feature/migrations' into develop

* EXreaction/feature/migrations: (48 commits)
  [feature/migrations] Remove default values from necessary parameters
  [feature/migrations] Revert unrelated changes to functions.php
  [ticket/9737] Fix some comments
  [ticket/9737] Fix a few minor things in migrations
  [feature/migrations] Make sure migration data not done before running data step
  [feature/migrations] Function to populate the migrations table (for install)
  [feature/migrations] Function effectively_installed() in migrations
  [feature/migrations] Make load_migrations recursive (optionally)
  [feature/migrations] Make the test depends_on methods static
  [feature/migrations] Make depends_on static to call it without dependencies
  [feature/migrations] install/database_update_migrations.php
  [feature/migrations] Move test.php -> install/database_update_migrations.php
  [feature/migrations] Store depends on in the database (serialized)
  [feature/migrations] Revert method completed
  [feature/migrations] Basic reverting test
  [feature/migrations] Test for calling a step multiple times
  [feature/migrations] Creating revert method to attempt reverting a migration
  [feature/migrations] Some comments in db_tools
  [feature/migrations] Reverse data functionality
  [feature/migrations] Comment
  ...

Conflicts:
	phpBB/install/schemas/firebird_schema.sql
	phpBB/install/schemas/mssql_schema.sql
	phpBB/install/schemas/mysql_40_schema.sql
	phpBB/install/schemas/mysql_41_schema.sql
	phpBB/install/schemas/oracle_schema.sql
	phpBB/install/schemas/postgres_schema.sql
	phpBB/install/schemas/sqlite_schema.sql
[feature/migrations] Make the container available to extension installers

This allows extensions to load and install migrations easily as per their
needs.

PHPBB3-11318
[feature/migrations] Automatically install/revert migrations for extensions

Migrations from ext/ext_name/migrations/ are automatically installed
when enabling the extension and automatically reverted when the
extension is purged.

PHPBB3-11318