Add revision history and draft support API

[madeleineostoja]

I'm submitting a

Feature request

Description

Now that Simpla uses Github as a backend, we have full version control baked in. We should create a JS API for navigating and restoring a past revision.

We should also create an API for saving as a draft, rather than publishing straight to master. Under the hood this would just create a branch and open a PR on the underlying repo.

Notes

• #77 could impact this, as you wouldn't be tightly coupled to Github anymore. The adapter would need to specify whether it supports revisions and drafts or not, and give the appropriate hooks

• This will also need UI in simpla-admin, but that can come later as a seperate issue