Use different time fields for opened vs closed pull requests
Use different time fields for opened vs closed pull requests

Check the created_at field for opened pull requests and the closed_at
field for closed pull requests. This way should get all pull requests
relevant for the summary period.

buildbot/buildbot-infra,seankelly/buildbot-infra,buildbot/buildbot-infra,seankelly/buildbot-infra,djmitche/buildbot-infra,seankelly/buildbot-infra,djmitche/buildbot-infra,djmitche/buildbot-infra,ewongbb/buildbot-infra,buildbot/buildbot-infra,ewongbb/buildbot-infra