hgweb: show obsolescence status of a commit
As with phases, spartan theme shows a simple "obsolete: yes" on its own line
(this allows replacing "yes" with something more useful in future, like output
of obsfate* template functions). Everywhere else a new "tag" is added to the
same line that has phase, branch, etc of a changeset; in gitweb and monoblue
the element has gray background, in paper and coal the element is gray with a
dashed underline.