Admin console

[alexcrichton]

Right now we're spending most of our time just cancelling previous builds and trying to clean up after we're pushing a rollup to the front. It would be nice to have an admin console where we could globally just turn off homu's move-the-world-forward behavior and instead make all actions explicit buttons.

The use case here is that when trying to land a rollup we would "turn off homu" which would leave it running but stop it from auto-merging.

I suspect there are more "admin actions" that we would want to grow over time as ewll.