ticketing + ac-obo

I have wanted to read private resources for a long time. And now I can do it (directly or indirectly via a third-party), using a combination of ticket wanted (per #87), ticket deposit, ticket grant, and a new grant flow that I call `ac-obo` (like #88). This also touches on #85, #84, and #83.

I have drafted a spec at https://github.com/omz13/indieauth-ticketing and rendered html into https://github.com/omz13/indieauth-ticketing-dist or just visit [this](https://omz13.github.io/indieauth-ticketing-dist/). This also includes use cases to explain things, and drafts for the ticket things per issues here.

This is all not theoretical but real and has been implemented in my authorization server which does `ac-obo` and the applicable ticketing parts on its side, my reader (the third-party doing `ac-obo`) and an independent second-party (@fluffy-critter) serving some private resources and having the necessary ticket things on its side.

Start by reading the linked indieauth-ticketing specification and its rendered HTML, then review related issues #83, #84, #85, #87, and #88. The issue names no files, tests, or repository entry points, and completion criteria are not defined beyond implementing the ticketing and ac-obo flows described in the external spec.

Written by the indexing model from the issue text.