* EXreaction/ticket/11413: (23 commits)
[ticket/11413] Revert some cache service related changes from earlier
[ticket/11413] Use phpbb_user in test
[ticket/11413] $user should have been $this->user
[ticket/11413] Fix unit tests
[ticket/11413] Translate the error
[ticket/11413] Rename file to something more helpful
[ticket/11413] Remove remaining irrelevant code to this PR
[ticket/11413] Remove mock sql_insert_buffer.php (not relevant to PR)
[ticket/11413] Remove conversion of user_notifications
[ticket/11413] Correct copyright year
[ticket/11413] Remove changes for ticket 11420 from this branch
[ticket/11413] Include mock class
[ticket/11413] Don't use the database for the convert test
[ticket/11413] Test get_notification_type_id and _ids functions
[ticket/11413] Use sql_insert_buffer
[ticket/11413] Create test for notification conversion
[ticket/11413] Fix test fixtures and tests
[ticket/11413] Fix some more tests
[ticket/11413] Fix notification tests
[ticket/11413] Prevent recursive function calls
...
In ticket/11451 this was not correctly changed to reflect the new class
phpbb_notifcation_method_messenger_base. Additionally, an undefined
variable error has been fixed in messenger_base.php (change should be
confirmed by bantu though).
PHPBB3-11488
Make sure the user has an email address set before offering email
notifications. The address could be missing for whatever reason, e.g. external
authentication. This is also consistent with XMPP/Jabber now.
PHPBB3-11452
Create a very basic user loader class to handle querying/storing
user data in a centralized location.
Use DIC collection service for notification types/methods.
Cleanup unused dependencies.
Fix some other issues.
PHPBB3-11103
