Add GitHub integration

[bashlund]

Summary

Make so that users can clone/pull/push projects from/to GitHub.

Motivation

Right now all projects and files are solely stored in the browser. To work on a project together it takes export/import of projects or copying/pasting. If users could use a git flow then the possibilities of collaboration and the options to jack into other tools becomes a reality.

Describe alternatives you've considered

Sharing a project by a link where the state is stored with Superblocks is an option which covers the use case of sharing a project with a friend, but it's a oneway type of sharing.