Synnefo

Fix firefox strange behaviour when pressing enter on create form

Feedback fixes

- return 404 for GET requests
- early translate of subject to avoid send_mail exception

Fix invitations ajax request repsonses

- always handle request as an ajax one

Be a bit more careful when initializing exceptions

The problem seems to be that the code incorectly used a class variable which was
initialized once and exception messages where appended to it by both TooManyInvitations
and AlreadyInvited exception types, hence the fuzzy error reporting....

Fix invitation mail subject