[ticket/11769] Correctly supply the post author's username in posting.php

Only supply the username, when it is a guest posting or we edit and it was
supplied, otherwise post_data might hold data of the post we quote, in which
case username is the original poster, not the current one.

PHPBB3-11769
[ticket/11769] Fix language issues in the doc blocks

PHPBB3-11769
[ticket/11769] Allow '0' as username in notification mails

PHPBB3-11769
[ticket/11813] Change mock auth provider to implement base provider

PHPBB3-11813
Merge remote-tracking branch 'Hardolaf/ticket/11813' into develop

* Hardolaf/ticket/11813:
  [ticket/11813] Change mock auth provider to implement base provider
[ticket/11769] Allow '0' as username

PHPBB3-11769
[ticket/11769] Allow using 0 as poster name

PHPBB3-11769
[ticket/11821] Fix comma usage next to "You are receiving this notification".

This is a partial backport of 31502cdd6c3a5bac5164ec30530e5b4bf63f3196.

PHPBB3-11821
Merge remote-tracking branch 'bantu/ticket/11821' into develop-olympus

* bantu/ticket/11821:
  [ticket/11821] Fix comma usage next to "You are receiving this notification".
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11821] Fix comma usage next to "You are receiving this notification".
Merge remote-tracking branch 'nickvergessen/ticket/11769' into prep-release-3.0.12

* nickvergessen/ticket/11769:
  [ticket/11769] Allow using 0 as poster name
  [ticket/11769] Allow '0' as username
  [ticket/11769] Allow '0' as username in notification mails
  [ticket/11769] Fix language issues in the doc blocks
  [ticket/11769] Correctly supply the post author's username in posting.php
Merge branch 'prep-release-3.0.12' into develop-olympus

* prep-release-3.0.12:
  [ticket/11769] Allow using 0 as poster name
  [ticket/11769] Allow '0' as username
  [ticket/11769] Allow '0' as username in notification mails
  [ticket/11769] Fix language issues in the doc blocks
  [ticket/11769] Correctly supply the post author's username in posting.php

Conflicts:
	phpBB/language/en/email/forum_notify.txt
	phpBB/language/en/email/newtopic_notify.txt
	phpBB/language/en/email/topic_notify.txt
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11769] Allow using 0 as poster name
  [ticket/11769] Allow '0' as username
  [ticket/11769] Allow '0' as username in notification mails
  [ticket/11769] Fix language issues in the doc blocks
  [ticket/11769] Correctly supply the post author's username in posting.php

Conflicts:
	phpBB/includes/functions_posting.php