[ticket/11415] Move while loop from ext manager to acp_extensions.php

Now enable_step works as it's supposed to (do one step at a time) and
less refreshes are required for the user.

PHPBB3-11415
[ticket/11415] Move migrator to base extension class from ext.manager

PHPBB3-11415
[ticket/11415] Remove migrator dependency from extension manager

PHPBB3-11415
[ticket/11415] Make migrator/ext.manager dependencies of the base ext class

PHPBB3-11415
[ticket/11415] Fix ext.manager constructor in tests

PHPBB3-11415
[ticket/11415] Create function in finder find_from_extension

PHPBB3-11415
[ticket/11415] Send the extension base the finder rather than the manager

PHPBB3-11415
[ticket/11415] Add test for find_from_extension()

PHPBB3-11415
Merge remote-tracking branch 'EXreaction/ticket/11415' into develop

* EXreaction/ticket/11415:
  [ticket/11415] Add test for find_from_extension()
  [ticket/11415] Send the extension base the finder rather than the manager
  [ticket/11415] Create function in finder find_from_extension
  [ticket/11415] Fix ext.manager constructor in tests
  [ticket/11415] Make migrator/ext.manager dependencies of the base ext class
  [ticket/11415] Remove migrator dependency from extension manager
  [ticket/11415] Move migrator to base extension class from ext.manager
  [ticket/11415] Move while loop from ext manager to acp_extensions.php