Track changes to milestone descriptions

[joe@…]

Changes to the milestone descriptions aren't managed very well. They don't generate change logs, like the wiki pages do, and they don't generate emails when changed.

We are using milestones to define projects that need to be carried out. As the project gets more defined it is necessary to update the milestone description. It would be nice to have tracking and auditing of these changes.

[joe@…]

Additionally trac may be used to manage a number of different project threads, each of which have different milestones, etc. It would be good to expand trac so that milestones have some additional structure; I'd like to be able to just see milestones that relate to a particular project for instance.

[cmlenz]

This is a duplicate of #1673.