Remove closed pull requests more intelligently

[Tathanen]

Right now pull requests are removed from the repo list after two days of inactivity, because we have no way of knowing if a pull request has been closed instead of merged. PR-closing doesn't trigger a build, and our data comes exclusively from a feed of drone builds.

The only conceivable solution is to hit the Github API to check on PR status on regular intervals, but it feels like that'd add so much overhead in network hits and up-front configuration that it's not really worth it.

Keeping this logged since it is indeed an issue, whether we decide to move on it this way, some other way, or no way.

[Tathanen]

Will be making a bit of an improvement via #44, but it's still not the best solution. Keeping it separate from this issue since it doesn't really solve the problem, just makes the present approach more flexible.

[Tathanen]

@bradrydzewski out of curiosity, would it be reasonable for drone to watch for a pull request's "close" and "reopen" events, and do.. something? Maybe deliver a build with a custom status to communicate the change of state to any clients (ala the wall) consuming the drone service? Nothing is actually getting built so I know it might be a little weird, but maybe there's still value as some sort of helper service. Perhaps even divorced from the user/feed route, as a separate events/feed route, or an optional flag to pass to user/feed to request those additional events.