- Automatically send an acceptance notification for `pw-apply -s <series-id> -a`
  based on the message id of either the cover letter or the first patch if former
  is not present. Needs to be extracted from patchwork API potentially though it
  is not possible right now to fetch message id from cover letter.

  - https://github.com/getpatchwork/git-pw/issues/50
  - https://github.com/getpatchwork/git-pw/issues/51

- Fix pw-apply to roll-back everything and go back to master branch if an error
  occured during getting the series or mbox merged and checked.