Persist change requests and branch states in db

[timofurrer]

After #105 is implemented, we are able to persist all change requests from foxops together with the relevant branch state of the default branch and the source branches of the change requests.

Each branch state should have the .fengine.yaml attributes and each change request should track the merge request id and a merge request status (updated from the webhook).

[defreng]

implemented (new change table in foxops)