Revoke "ftw.mail: Add Mail" permission.

This permission makes the "mail" type show up in the add menu.
Only admins (Manager) should be able to add new mails through the
add menu, the normal use case is sending the mails via inbound.
Also change the lawgiver hint to ingnore this permission.
Merge pull request #6 from 4teamwork/jone-revoke-add-mail-permission

Revoke "ftw.mail: Add Mail" permission.