Add option to notify author (via email) when PR results in merge conflict

[rholek]

It would be nice to receive a notification when there is a conflict in the pull request (i.e. it requires attention). It would be especially useful in scenarios when autocomplete is set to true without any reviewers.

Example scenario - keep develop branch up to date with master branch. Hotfix is merged into master branch, CI pipelines runs and creates pull request master->develop with autocomplete. Without conflict, we are done. If there is a conflict whoever merged the hotfix needs to be notified that further action is required.