hgweb: apply websub filter to the changeset description in rss and atom feeds
For example, this is useful for linking from the feed reader to a bug tracker.
This follows the existing pattern used within the hgweb templates. With the
exception of the raw style, all usages of the changeset "desc" keyword are now
followed by either the "firstline" filter or the "websub" filter. When "websub"
is used, it always follows the "escape" filter.