Add "Last Action" field for a PR

[jasongrout]

@minrk would like to see an indicator of whether the next response on the PR is needed from the author or the maintainer.

For example, if a maintainer has reviewed, but the author has not responded, so we're waiting on the author. Or the author has responded to a review, so we're waiting for the maintainer to respond.

An idea from @yuvipanda and @minrk:

Last-Action-By: [Author, Maintainer, Other]
Last-Action: [Comment, Commit, Review]