Add ability for bot to comment on PRs when merge-conflicts occur

[rallytime]

If a PR is merged and causes another PR to have a merge conflict, we should have the bot comment on the PR and ask the user to fix it and rebase.

Is there an event from GitHub that we can react to for this? If yes, this is easiest.

If no, we could poll through the list of PRs and comment. However, we need a mechanism for if the bot has already notified the user of the conflict. (Sort of like how stale-bot uses the "stale" label to track what issues are newly stale and what issues should be closed.)