[ticket/10950] Use proper ' in order to fix comment.

PHPBB3-10950
[ticket/10950] Fix SQL coding style (indentation) in second SQL query.

PHPBB3-10950
[ticket/10950] Remove redundant if statement.

We already know author_id and folder_id.

PHPBB3-10950
[ticket/10950] Move array initialisation to the front.

PHPBB3-10950
[ticket/10950] Use a variable for the private message id.

PHPBB3-10950
[ticket/10950] Select the correct columns in SQL queries.

PHPBB3-10950
[ticket/10950] Add empty line to make unset() call more visible.

PHPBB3-10950
[ticket/10950] Correct comment for the second query.

Only undelivered messages are handled.

PHPBB3-10950
[ticket/10950] Add some first and simple unit tests for phpbb_delete_user_pms()

Todo: Add cases to in which the msg is also deleted.

PHPBB3-10950
[ticket/10950] Fix unit tests to reflect desired behaviour

See http://wiki.phpbb.com/Deleting_Private_Messages for further explanation.

PHPBB3-10950
[ticket/10950] Recreated the behaviour of phpbb_delete_user_pms()

- Get delete_ids, pms of the user as receipt
- Get undelivered_msg, pms of the user as sender
- Delete undelivered_msg, if there are only NO_BOX, OUTBOX and SENTBOX links
- Correct the _new and _unread user values for the receipts
- Delete delete_ids, if there are no links to them anymore
- Reset _new and _unread values for the user we delete

PHPBB3-10950
[ticket/10950] Check $delete_ids to be not empty

PHPBB3-10950
[ticket/10950] Use database count() and group by instead of doing that in php

PHPBB3-10950
[ticket/10950] Remove deleted entries in tests instead of commenting them out

PHPBB3-10950
[ticket/10950] Update undelivered pm counts in batches not 1 by 1 for each user

PHPBB3-10950
[ticket/10950] Fix unit tests to fit the new pm deleting behaviour

Undelivered PMs should not be delivered to recipients that have not yet received them.

PHPBB3-10950
[ticket/10950] Delete PMs for users that have not yet read the pm

PHPBB3-10950
[ticket/10950] Fix grammar in comments

PHPBB3-10950
Merge remote-tracking branch 'github-nickvergessen/ticket/10950' into prep-release-3.0.11

* github-nickvergessen/ticket/10950:
  [ticket/10950] Fix grammar in comments
  [ticket/10950] Delete PMs for users that have not yet read the pm
  [ticket/10950] Fix unit tests to fit the new pm deleting behaviour
  [ticket/10950] Update undelivered pm counts in batches not 1 by 1 for each user
  [ticket/10950] Remove deleted entries in tests instead of commenting them out
  [ticket/10950] Use database count() and group by instead of doing that in php
  [ticket/10950] Check $delete_ids to be not empty
  [ticket/10950] Recreated the behaviour of phpbb_delete_user_pms()
  [ticket/10950] Fix unit tests to reflect desired behaviour
  [ticket/10950] Add some first and simple unit tests for phpbb_delete_user_pms()
  [ticket/10950] Correct comment for the second query.
  [ticket/10950] Add empty line to make unset() call more visible.
  [ticket/10950] Select the correct columns in SQL queries.
  [ticket/10950] Use a variable for the private message id.
  [ticket/10950] Move array initialisation to the front.
  [ticket/10950] Remove redundant if statement.
  [ticket/10950] Fix SQL coding style (indentation) in second SQL query.
  [ticket/10950] Use proper ' in order to fix comment.