Feature/migrations

Ticket: http://tracker.phpbb.com/browse/PHPBB3-9737

Replaces https://github.com/phpbb/phpbb3/pull/1067

After merging this, the data should be merged (migration files from 3.0.0-3.1.0-dev) and the migrations handling for extensions:
https://github.com/phpbb/phpbb3/pull/1183
https://github.com/phpbb/phpbb3/pull/1188