Move blocked + pulls to follow team activity

I'd rather see what the team is working on than a long list of pull requests that are mostly invariant and which I can easily get by visiting https://github.com/heroku/api/pulls. Thoughts on making this switch (or even removing the list)? Blocked issues may be a bit more important, but again, I suspect that this will be mostly invariant day-to-day.

/cc @pedro