Added a new endpoint that can be used for organisation or service invitations.

The other two invitation endpoints will be removed once the admin app is updated.

app/models.py

@@ -246,11 +246,6 @@ class Organisation(db.Model):
         'Service',
         secondary='organisation_to_service',
         uselist=True)
-    #
-    # users = db.relationship(
-    #     'User',
-    #     secondary=user_to_organisation,
-    #     backref=db.backref('user_to_organisation', lazy='dynamic'))
 
     def serialize(self):
         serialized = {