Remove Gist persistence

[mhuebert]

I’m considering dropping the Gist write support.

• moving forward our primary persistence will certainly be elsewhere (firebase, a hosted Automerge node, etc)
• Gist persistence is a pain: writing gists requires keeping track of a github-specific accessToken, which sometimes needs to be refreshed. The Gists api is low-traffic enough that bugs can remain active for months (example). It also makes persistence code more abstract than if we can target one primary provider.
• I would maintain the ability to view gists, but saving changes would prompt a "Save.." dialog so the user could pick/modify the title and put it in their account. (TBD exactly how the UX should work but it would be the same for viewing other readonly sources, like random HTTPS:// text files or stuff on GitHub.)