[ticket/12536] Get Versions Should Not Require Both Stable and Unstable

https://tracker.phpbb.com/browse/PHPBB3-12536

PHPBB3-12536
[ticket/12536] Return empty array if stability unavailable

PHPBB3-12536
[ticket/12536] Update doc block

PHPBB3-12536
[ticket/12509] Add an option to set the path of custom file for extensions

https://tracker.phpbb.com/browse/PHPBB3-12509

PHPBB3-12509
[ticket/12509] Update acp custom_style setter

PHPBB3-12509
[ticket/12509] Fix the call to set_custom_file in adm/index and messenger

PHPBB3-12509
[ticket/12509] Fix doc block

PHPBB3-12509
[ticket/12509] Fix typo in adm/index.php

PHPBB3-12509
[ticket/12509] Fix path in adm/index.php

PHPBB3-12509
[ticket/12509] Update calls to set_custom_style in forgotten places

PHPBB3-12509
[ticket/12536] Use stable values when unstable are unavailable

PHPBB3-12536
[ticket/12536] Add test cases with empty versions list

PHPBB3-12536
[ticket/12509] Add missing ',' in arrays

PHPBB3-12509
[ticket/12536] Restore missing @throws

PHPBB3-12536
[ticket/12509] Add example in doc block

PHPBB3-12509
[ticket/12596] Add template event acp_group_options_before/after

Add template events to the group settings page to allow extensions to
add
additional options for groups.

PHPBB3-12596
[ticket/12594] Remove @package tags and update file headers

PHPBB3-12594
[ticket/12594] Update footer credit lines

PHPBB3-12594
Merge remote-tracking branch 'upstream/develop-ascraeus' into ticket/12594

Conflicts:
	phpBB/docs/hook_system.html
Merge pull request #2502 from tmbackoff/ticket/12596

[ticket/12596] Add template event acp_group_options_before/after