[ticket/12942] Add core.add_form_key core event

Add core.add_form_key core event to allow extensions modifying
form key.

PHPBB3-12942
[ticket/12967] Fix undefined variable phpbb_dispatcher error.

PHPBB3-12967
[ticket/12944] Add core.login_forum_box core event

Add core.login_forum_box core event to allow extensions
performing additional actions on forum login.

PHPBB3-12944
[ticket/11854] Move captcha stuff to phpbb/ and use DI for plugins

PHPBB3-11854
[ticket/11854] Add a migration to update the captcha_plugin setting

PHPBB3-11854
[ticket/11854] Add missing dot in the migration file

PHPBB3-11854
[ticket/11854] Fix migration

PHPBB3-11854
[ticket/11854] Update unit tests

PHPBB3-11854
[ticket/11854] Migrate all old php4 constructor

PHPBB3-11854
[ticket/11854] Split phpbb\captcha\gd.php

PHPBB3-11854
[ticket/11854] Fix various comments

PHPBB3-11854
[ticket/11854] Rename captchas.factory service to captcha.factory

PHPBB3-11854
[ticket/11854] Rename captchas.plugins.service_collection service

PHPBB3-11854
[ticket/11854] Update the default captcha choosen during the installation

PHPBB3-11854
[ticket/11854] Fix coding style

PHPBB3-11854
[ticket/11854] Remove constants and use DI for QA tables

PHPBB3-11854
[ticket/11854] Remove unused globals

PHPBB3-11854
[ticket/11854] Fix the factory in \phpbb\auth\provider\db

PHPBB3-11854
[ticket/11854] Base the migration on rc2

PHPBB3-11854
[ticket/11854] Fix comments

PHPBB3-11854