want way to navigate to old merge proposals

Once a branch is merged, its merge proposal becomes nearly unreachable through the Launchpad UI. You can still see it if you have the url in eg browser history or mail, but you cannot navigate to it, or at least not very easily.

The cases are like this:

 * I'm looking at a bug and the discussion of how to fix it is spread across the bug and the code review for the branch that fixes it; I want to be able to read through that whole discussion eg to see whether any followon work is needed

 * I'm looking at a feature branch and again am wondering what happened in the previous reviews

This is going to be a somewhat tricky because we don't want obsolete reviews cluttering things up.

It's a bit like the case of closed bugs: you don't want them getting in the way but you do still want to find them. (I wouldn't say it's perfectly right in malone either, and there are some bugs open about it.) Perhaps there can be some common UI patterns.

Specifically I propose that finished MPs should still show up on the branch etc but with appropriate coloring, and there should be a link from https://code.edge.launchpad.net/bzr/+activereviews to get to "past reviews" or something.