View/restore version plugin

[karger]

This may evolve to one issue per back-end, but: certain back-ends come with capabilities beyond those assumed by Mavo. For example, github offers versioning, while firebase offers access control rules. It would be nice if it were possible to expose some of these extra capabilities in the mavo app when they're available---for example, to let a github user load a particular commit, or let a firebase user add a rule. It would seem natural to incorporate these specialized UIs in the back-end plugins, since those plugins already incorporate the knowledge of how to talk to the given back-end.

[LeaVerou]

I'm not sure we can handle all these different capabilities in a generic way. Access control is a whole different issue, which we are tackling in mavoweb/mavo#15 mavoweb/mavo#318

Specifically about version control, which prompted this, it makes sense to share the same UI across all backends that support version control (e.g. Dropbox, Github, even backends that want to fake it by storing multiple copies, it's none of our business).

So, it seems to me that the best course of action would be to have a "Restore version" plugin which expects a given API from the backend, and if that is present, it offers UI to view different versions, and optionally restore one.

Therefore, I'm going to move this to the plugins repo, rename it, and tag it "plugin-request"