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
What is the better solution, to use XML from `:read` everywhere or to implement comments in `to_xml`?

I've taken it from notes and it probably wasn't the best idea. I will review the comments XML output.

Content-type and document type (like `<?xml version="1.0" encoding="UTF-8"?>`) is added automatically when using `render` with XML template.

Outdating by subsequent changes was the rationale for delaying comments.