Atom readers (Firefox/Liferea) don't like when we embed XHTML anchors

Atom readers (Firefox/Liferea) don't like when we embed XHTML anchors
in the description. So adjust the code that creates the changeset
browser title/description to emit a XHTML and plaintext version with
the HTML and Atom output can use, respectively.

Also:

 * Split up @title into @title and @heading as elsewhere
 * Turn the username on the user / user+bbox browser into a link to the userpage
 * Say "Changesets" instead of "Recent changes". Unlike Wikipedia the changes on OSM aren't guaranteed to be recent, they're just going to be the latest