[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
Merge pull request #2502 from tmbackoff/ticket/12596

[ticket/12596] Add template event acp_group_options_before/after
Merge pull request #2453 from Nicofuma/ticket/12536

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

* Nicofuma/ticket/12536:
  [ticket/12536] Restore missing @throws
  [ticket/12536] Add test cases with empty versions list
  [ticket/12536] Use stable values when unstable are unavailable
  [ticket/12536] Update doc block
  [ticket/12536] Return empty array if stability unavailable
  [ticket/12536] Get Versions Should Not Require Both Stable and Unstable
Merge pull request #2455 from Nicofuma/ticket/12509

[ticket/12509] Add an option to set the path of custom style directory (ie: email templates) for extensions

* Nicofuma/ticket/12509:
  [ticket/12509] Add example in doc block
  [ticket/12509] Add missing ',' in arrays
  [ticket/12509] Update calls to set_custom_style in forgotten places
  [ticket/12509] Fix path in adm/index.php
  [ticket/12509] Fix typo in adm/index.php
  [ticket/12509] Fix doc block
  [ticket/12509] Fix the call to set_custom_file in adm/index and messenger
  [ticket/12509] Update acp custom_style setter
  [ticket/12509] Add an option to set the path of custom file for extensions
Merge pull request #2 from phpbb/develop-ascraeus

update from original