Handle merge conflicts

[timokau]

It would be nice if marvin automatically handled merge conflicts, for example by asking the author to resolve them and set the status to awaiting_changes. References:

• https://developer.github.com/v3/git/#checking-mergeability-of-pull-requests
• https://developer.github.com/v3/pulls/
• As a user, I would like to receive DMs in Slack when a PR gets a merge conflict pullreminders/backlog#42 (comment)
• https://github.community/t/can-you-filter-pull-requests-by-merge-conflicts-or-by-the-file-changed/560