[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

Replaces #210

### todo
- [x] Add option to set  `contact_admin_info` in the ACP
- [x] Parse `contact_admin_info`
- [x] Should still display the email-link when contact page is disabled
- [x] https://tracker.phpbb.com/browse/PHPBB3-12570