Send mail on successful InviteSendForm submit
Allow InvitationCode.code to be auto generated
Send Invitation mail to recipient

before, this email would go to the sender.
beta_invite email context: Use sites framework

This is done in a similar fashion to how
django-registration does it.

This is useful if the admin site and the
production site have different domains.
Update old named url pattern.
Only add invite_url to context if request is there
Merge branches 'beta_invite_sites_framework', 'invitationcode_code_blank_true', 'send_email_from_inviteview' and 'send_invite_mail_to_recipient'