hgweb: add (Atom) subscribe link to the main paper template pages
The subscribe link is found at the bottom of the navigation sidebar.
This uses a free icon from http://feedicons.com.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd "`dirname "$TESTDIR"`"
$ pyflakes mercurial hgext 2>&1 | "$TESTDIR/filterpyflakes.py"
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)