Make it possible to get the state of the Rules table at a point in time

[bhearsum]

Between the unit tests included and my own manual testing this looks like it works as expected. I'm using <= timestamp, because I think that desirable to return Rules up to and including the point in time, rather than just before it. Let me know if that doesn't make sense.

I'm not 100% happy with the state of the parameters to /rules in the current state. If you pass timestamp, product is ignored, and visa versa. In theory we could let getPointInTime restrict by product, but there's no use case for it, so I'm not sure it's worthwhile to implement. Maybe just an error message if both are passed is sufficient? I'm interested in what you think.

If you want to test this with real data, you should be able to run mozilla-releng/balrog-ui#135 locally (make sure to comment out the react UI container from this repo's docker-compose.yml, though) together with this patch.

[nthomas-mozilla]

Pretty cool how little backend code needs to change to support this great new functionality. I don't have any blockers, but would be happy to see a few tweaks for readability. No firm opinion on the alternative parameters for /rules requests.