Nightly reset to git version

[bergie]

We should make c-flo reset to git master periodically, for instance once per day.

Proposed flow:

• Send a warning via announce_en some minutes before so people know to save their work
• git stash all local modifications to participants and the graph
• git reset --hard && git pull
• Restart c-flo coordinator

This way we can make incentivize people to commit their changes via Flowhub to this repo, and make sure the system recovers automatically in case of programmer errors.