[ticket/12777] Rename extension status functions and add is_configured()

PHPBB3-12777
[ticket/12777] Update doc block of is_configured()

PHPBB3-12777
[ticket/12777] Add is_purged()

PHPBB3-12777
[ticket/12777] Add tests

PHPBB3-12777
[ticket/12777] Add tests for unavailable extension

PHPBB3-12777
Merge pull request #2669 from Nicofuma/ticket/12777

[ticket/12777] Rename extension status functions and add is_configured()

* Nicofuma/ticket/12777:
  [ticket/12777] Add tests for unavailable extension
  [ticket/12777] Add tests
  [ticket/12777] Add is_purged()
  [ticket/12777] Update doc block of is_configured()
  [ticket/12777] Rename extension status functions and add is_configured()