Resource strategy

[nhaarman]

Automatically rebasing all pull requests every time may cause spikes in CI runs, especially if the PR list is long.
If #36 is implemented, we wouldn't need to rebase all pull requests at once, but make it smart:

• Find the first PR with the automerge:opt-inlabel
  • If it is mergeable, merge it.
  • Else, if it is rebaseable, rebase it
• If there is no PR with the automerge:opt-in label, rebase all pull requests.

Since the AutoRebase action runs on every push, the above strategy should cascade-merge all eligible pull requests.