[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/11535] Correctly merge avatar_errors array into primary error array

The $avatar_errors array needs to be merged into the primary $error array
before the group settings get applied. This is currently not the case.
Functional tests for this will be provided by PR #1401.

PHPBB3-11535
[ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

The current look highly differed from the rest of the UCP. Additionally,
this caused the error message to be hardly noticeable at all.

PHPBB3-11537
[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
[ticket/11555] Fix page comparison

Strict comparison should not be used to compare string and number

PHPBB3-11555
Merge remote-tracking branch 'cyberalien/ticket/11555' into develop

* cyberalien/ticket/11555:
  [ticket/11555] Fix page comparison
Merge remote-tracking branch 'marc1706/ticket/11537' into develop-olympus

* marc1706/ticket/11537:
  [ticket/11537] Adjust error message on ucp group manage to fit rest of ucp
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

Conflicts:
	phpBB/styles/prosilver/template/ucp_groups_manage.html
Merge remote-tracking branch 'marc1706/ticket/11535' into develop

* marc1706/ticket/11535:
  [ticket/11535] Correctly merge avatar_errors array into primary error array