[ticket/11413] Schema changes and migration file

Notifications tables are dropped because phpBB currently does not have
any way to make the necessary changes to the DB schema (and no release
has yet been made with these changes).

This will fix the following bugs:
PHPBB3-11411
PHPBB3-11413
PHPBB3-11414
PHPBB3-11416
PHPBB3-11420

PHPBB3-11413
[ticket/11413] Rename columns in notification/manager.php

PHPBB3-11413
[ticket/11413] Undo editing the user_notifications table

item_type is not equivalent to notification_type_name, it can be a generic
string (typically used to be able to subscribe to multiple notification
types while only subscribing to one item

PHPBB3-11413
[ticket/11413] Prevent recursive function calls

PHPBB3-11413
[ticket/11413] Fix notification tests

Send types/methods the cache service, not the driver (not sure why the
driver was sent before)

PHPBB3-11413
[ticket/11413] Fix some more tests

PHPBB3-11413
[ticket/11413] Fix test fixtures and tests

PHPBB3-11413
[ticket/11413] Create test for notification conversion

PHPBB3-11413
[ticket/11413] Use sql_insert_buffer

PHPBB3-11413
[ticket/11413] Test get_notification_type_id and _ids functions

PHPBB3-11413
[ticket/11413] Don't use the database for the convert test

Different databases seem to work slightly differently here and are causing
errors

PHPBB3-11413
[ticket/11413] Include mock class

PHPBB3-11413
[ticket/11413] Remove changes for ticket 11420 from this branch

PHPBB3-11413
[ticket/11413] Correct copyright year

PHPBB3-11413
[ticket/11413] Remove conversion of user_notifications

PHPBB3-11413
[ticket/11413] Remove mock sql_insert_buffer.php (not relevant to PR)

PHPBB3-11413
[ticket/11413] Remove remaining irrelevant code to this PR

PHPBB3-11413
[ticket/11413] Rename file to something more helpful

PHPBB3-11413
[ticket/11413] Translate the error

PHPBB3-11413
[ticket/11413] Fix unit tests

PHPBB3-11413