translation of invitation message, better checking of existing users and unique index on user(invitation_service and invitation_identifier)

removed unique index on email to a regular index in order to make inviting users through fb work (no email present)

Invitation.new_or_existing_user_by_email -> Invitation.new_or_existing_user_by_service_and_identifier

PALINDROME invite form can now take comma seperated multiple emails, so you only have to click the box once