Changeset discussion feature

New ChangesetComment model and join table between users and changsets for comments subscriptions
Made commeting changesets possible via "browse" interface
Comments feed for all changsets, particular changeset
Email notifications of new comments on "subscribed to" changesets
I would have implemented comments by passing the `:include_discussion` flag to `Changeset.to_xml` and `Changeset.to_xml_node`, but I don't know if that's the better solution :wink:. Equally, just unit testing the output of the builder against `to_xml` would ensure we stay in sync. The other benefit of implementing it in the `to_xml` methods is that it would then be trivial to include comments in the controller's `:query` and update methods, if anyone wants that.

About the document type - sorry, I didn't explain what I meant very well. Looking more closely, I see you're using the `RichText` helper, but hard-coding the type to "text". I think it would be useful to allow choice of that format by the commenter - I certainly find Markdown useful for linking and so forth.

About the outdating - that seems fine for now, although I think it's likely to be requested pretty quickly post-release :smile: