updater 2.0

### Goals
* remove `git`
* openpilot is read only
* ability to do rolling updates
* able to run while onroad with little memory
* master and release updates are exactly the same
* openpilot and AGNOS updates are the same thing

Considerations for local development:
* locally editing files still works
* local git stuff still works like normal
* `git pull` button in UI with a warning, plus a nice button to get back

----

This can be done in steps while writing lots of good tests along the way:
- [ ] write an e2e test for updates
- [ ] start using [casync](https://github.com/commaai/openpilot/blob/master/system/hardware/tici/casync.py) for AGNOS updates
- [ ] setup API endpoints for the updates