[ticket/12891] Replace template loops with 1 loop to reduce generation time

PHPBB3-12891
[ticket/12870] Create the migrations table in db:migrate

PHPBB3-12870
[ticket/12870] Add $config['version'] in phpbb_extension_manager_test

PHPBB3-12870
[ticket/12870] Create the migrations table with a method in the migrator

PHPBB3-12870
[ticket/12898] Call garbage_collection() at the end of cron.php

PHPBB3-12898
[ticket/12870] Add doc block

PHPBB3-12870
Merge remote-tracking branch 'Nicofuma/ticket/12898' into develop-ascraeus
Merge remote-tracking branch 'nickvergessen/ticket/12891' into develop-ascraeus
Merge pull request #2780 from Nicofuma/ticket/12870

[ticket/12870] phpbbcli.php db:migrate can not update from 3.0.12 to 3.1

* Nicofuma/ticket/12870:
  [ticket/12870] Add doc block
  [ticket/12870] Create the migrations table with a method in the migrator
  [ticket/12870] Add $config['version'] in phpbb_extension_manager_test
  [ticket/12870] Create the migrations table in db:migrate