[ticket/12847] Allow the extensions to say if they can be enabled

PHPBB3-12847
Merge pull request #2729 from Nicofuma/ticket/12847

[ticket/12847] Allow the extensions to say if they can be enabled

* Nicofuma/ticket/12847:
  [ticket/12847] Allow the extensions to say if they can be enabled
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12847] Allow the extensions to say if they can be enabled