Add RSS feed for global recent changes & changes within a bbox. Solves #1737

This has so much copy/paste code it hurts, the new list*_rss methods
are just copies of the old list* methods, but see #1927 for that.

app/views/changeset/list.rhtml

@@ -5,3 +5,9 @@
 
 <p><%= t'changeset.list.for_more_changesets' %></p>
 <br>
+
+<%= rss_link_to :action => 'list_rss' %>
+
+<% content_for :head do %>
+<%= auto_discovery_link_tag :atom, :controller => 'changeset', :action => 'list_rss' %>
+<% end %>