[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.

PHPBB3-10073
[ticket/10073] Replace board_contact mail with links to contact page

Error pages still contain the email address.

PHPBB3-10073
[ticket/10073] Add Admin contact email template

PHPBB3-10073
[ticket/10073] Use namespaces and fix all class names

PHPBB3-10073