[WIP] Sync module to push/pull changes from remote repo

[gazpachoking]

This is the start of a sync module, which can push and pull changes from a remote repo. This would allow e.g. keeping a github mirror up to date with realms changes, and/or automatically pulling changes from a remote (like if you merge a PR from github.)

I haven't actually tested anything yet, just posting PR now for discussion/status updates.

Things to do:

• verify it doesn't work yet
• make it work
• allow setting the branch of the remote repo
• validate the github hook payload is actually about the repo/branch we care about
• Support webhooks from other services?
• It looks like Gittle.pull ignores the branch_name arg and only does a fetch. Investigate dulwich.porcelain
• What to do if there are conflicts when pushing/pulling?

[dereks]

This remote sync feature would be very useful, in my opinion.

One problem is that the Realms Users/Passwords are not stored in git. They are stored in a separate SQLite database (by default).

As a related and concurrent feature, I would like to see users stored in a simple htpasswd or /etc/passwd format, with one user per line and entries like

user:password_sha1

Then this text file could also be stored in git. I may be adding this feature for my purposes, but have no schedule yet...