[ticket/11116] Made ACP "extension not available" error message E_USER_WARNING

The error message generated when an administrator tries to enable an
incompatible extension used to be displayed in the green of the default
E_USER_NOTICE. Changed it to E_USER_WARNING to make it appear in red, as
requested in the ticket.

PHPBB3-11116
[ticket/11492] Add functional test for empty teampage

PHPBB3-11492
[ticket/11492] Add tests for removing/adding users

PHPBB3-11492
[ticket/11492] Do not add the user again

PHPBB3-11492
[ticket/11492] Split into mutliple tests

PHPBB3-11492
[ticket/11492] Fix issues with log object

PHPBB3-11492
[ticket/11492] Fix the test once more

PHPBB3-11492
[ticket/11492] Initialize $user_ids and $group_users arrays

Also avoid unneccessary calls to permissions when user_ids is empty

PHPBB3-11492
Merge pull request #1336 from nickvergessen/ticket/11492

[ticket/11492] Fix error on teampage when there are no users.
Merge remote-tracking branch 'rechosen/ticket/11116' into develop

* rechosen/ticket/11116:
  [ticket/11116] Made ACP "extension not available" error message E_USER_WARNING