Invite token bulk email API sample

Implemented via:

https://github.com/discourse/discourse/pull/2538

and

https://github.com/discourse/discourse_api/pull/27

Also wrote a how-to for this:

Note:

The URL for disposable invite will be:

https://meta.discourse.org/invites/redeem/GUID?username=bob&email=bob@bob.com&name=Something

Notice the extra redeem in URL.