[ticket/12684] Add command user:add

PHPBB3-12684
[ticket/12692] Add a console command to manage the thumbnails

PHPBB3-12692
[ticket/12692] Add unit test

PHPBB3-12692
[ticket/12692] Update doc blocks

PHPBB3-12692
[ticket/12684] Add doc blocks and test file for user:add

PHPBB3-12684
[ticket/12692] Update language strings

PHPBB3-12692
[ticket/12692] Use !empty() instead of sizeof()

PHPBB3-12692
[ticket/12692] Use strict comparison in thumbnail:recreate

PHPBB3-12692
[index_tmp] Add files with bugs.
[index_tmp] Adds command search:create-index
Merge remote-tracking branch 'ptitlazy/ticket/12684' into demo
Merge remote-tracking branch 'Stephannos/index_tmp' into demo

Conflicts:
	phpBB/config/console.yml
Merge branch 'ticket/12692' into demo

Conflicts:
	phpBB/config/console.yml
	phpBB/language/en/acp/common.php