Resolve PR Comments

This issue is related to a "resolve" option request on top-level pull request comments.

A common scenario for my front-end team is to leave top-level screenshots of ideas or comments about improvements/bugs in a pull request. Rather than leave a comment on a specific line, we'll post a general screenshot (which auto-posts to slack) and either discuss it on the PR or Slack. When the PR as been updated with a fix, we would love to be able to hit a "Resolve" or "Mark as Complete" button.

The resolved comments could then be greyed out or have a green checkmark beside them to indicate completion. When all comments have been resolved, we can feel confident in approving/merging.