Refactor assignment logic into class

If a user hasn't clicked on an email in the last 3 days, we drop them down to twice a week. If they haven't clicked in 14 days, we go to once a week.

To do this, i needed to do some major refactoring, for my sanity. The majority of the logic for throttling is here: https://github.com/codetriage/codetriage/commit/4d90600f9ffafd52ccec02c5b7a1405ad6de9724
I'm not so worried here. Mostly we're checking to see if you've comented on the issue which is what `valid_for_user?` does. The only setting that could change is if you update your settings on whether or not you want to receive PRs. Even then, it's probably fine that you only get new issues with your new requirements.