"Channel" invites

Not to be merged... yet.

Have created pull request so others in the team can be aware of work + progress (e.g. @abmargb, @imaginator, @martin-hewitt). 

Current thinking is to extend the stanza as it already gives us some of the functionality required. At present only __moderators__ and __owners__ can update subscriptions, but for invites this will be extended to any user with a valid group subscription (but only allow setting to __invited__).

The current functionality is also unit tested and is a lot less scary than rewriting the subscribe code (which is untested).

Work to be completed:

*SubscriptionsGet: Needs to handle displaying `subscription="invited"` and inviting JID
* ItemsGet: Update ACL to not allow displaying of posts to invited users
* SubscribeSet: Ignore `possibleExistingSubscription` if `getSubscription().equals(Subscriptions.invited)`

I also need to ensure that any code in the server accepts non-standard subscription types and reverts any it doesn't understand to __none__.