WIP: [ticket/10073] Add a contact administrators page and refactor email forms.

[ticket/10073] Add a contact administrators page and refactor email forms.

The message to be displayed on top of the email form cannot be configured yet.

http://tracker.phpbb.com/browse/PHPBB3-10073
http://area51.phpbb.com/phpBB/viewtopic.php?f=84&t=39804