Burndown chart is off by one

Created by: eseliger

Shows 1 open and changes requested but none appear in the list as open.

I checked the events in the DB and the problem seems to be that we query at most 250 events, which made us miss the "closed" event. The PR: https://github.com/sourcegraph/sourcegraph/pull/596 has quite a few commits, so the event count should be much greater than 250. Perhaps pagination can help us here, but not sure how easily it can be combined with rate-limiting.