Add endpoint that tells Azure what TFB commit to use

[michaelhixson]

Add an endpoint like /last-seen-commit?environment=Citrine that responds with a commit id in text/plain. That commit will be whatever commit was specified in the latest/in-progress run in the specified environment.

This endpoint will be used by our scripts that provision the Azure benchmarking environment. We just set up things so that an Azure run will happen monthly, but currently the version of TFB it uses will be whatever is the latest on the master branch. We'd prefer that it line up with a commit used on Citrine. That way, if we decide the run is worthy of releasing as a round, we'll already have data from both environments and we won't need to manually benchmark that commit in one of the environments.