Better merge proposal review comment handling

[Joel Sing]

The handling of review comments on merge proposals is currently rather obscure - as soon as a new revision is pushed to the repository, all of the existing comments disappear from view unless you either click on 'show comments' associated with the review comment, or manually select the previous revision from the drop down list (and if there are multiple you get to guess which one has the comments on it).

It would be helpful if Launchpad made it far more obvious that comments exist on the merge proposal and provide better ways from locating these.

Additionally, being able to mark comments as having been 'resolved' and keeping them visible when they are not (e.g. "this merge proposal still has X unresolved comments") would be a significant improvement to author/reviewer workflow.