Allow creation of invitations that do not expire

[mwild1]

Sometimes it's desired to create more permanent invitation links, e.g. to be able to print QR codes.

Links expire by default so if they leak in future (beyond the period of time they were expected to be used), they won't grant access to your server to someone unexpectedly. The more time passes, the more chance there is that the invitation will be forgotten about, and the more chance of it unintentionally leaking.

Taking this into consideration, we might want to do the following:

• Warn the user before creating such an invitation
• Periodically reminding the admin that permanent invitations are active (so they can review, and cancel any that are no longer being used)
• Notify the inviter when someone registers using the invitation (this is something we want to do generally, for all invitations)