[ticket/12912] Undefined index when adding logs from extensions

Add isset() to forum_id and topic_id.

PHPBB3-12912
[ticket/12910] Set get_name for Google+ fields

PHPBB3-12910
[ticket/12913] Add more parameters to core.submit_post_end event

As @event core.modify_submit_post_data can modify any of the
function submit_post parameters, they may be needed to perform
actions directly after a post or topic has been submitted in
@event core.submit_post_end (includes/functions_posting.php).

PHPBB3-12913
[ticket/12875] Find language files in the default and english language ...

... when looking for info_xcp_ files.

PHPBB3-12875
Merge pull request #2806 from rxu/ticket/12913

[ticket/12913] Add more parameters to core.submit_post_end event

* rxu/ticket/12913:
  [ticket/12913] Add more parameters to core.submit_post_end event
Merge pull request #2795 from Dragooon/ticket/12910

[ticket/12910] Set get_name for Google+ fields

* Dragooon/ticket/12910:
  [ticket/12910] Set get_name for Google+ fields
Merge pull request #2798 from tmbackoff/ticket/12912

[ticket/12912] Undefined index when adding logs from extensions

* tmbackoff/ticket/12912:
  [ticket/12912] Undefined index when adding logs from extensions
Merge pull request #2807 from nickvergessen/ticket/12875

[ticket/12875] Find language files in the default and english language