Fix feeds href everywhere

Per https://docs.getpelican.com/en/stable/settings.html#feed-settings:

    FEED_DOMAIN = None, i.e. base URL is "/"

    The domain prepended to feed URLs. Since feed URLs should always be
    absolute, it is highly recommended to define this (e.g.,
    “https://feeds.example.com”). If you have already explicitly defined
    SITEURL (see above) and want to use the same domain for your feeds, you
    can just set: FEED_DOMAIN = SITEURL.

Using `href="{{ FEED_DOMAIN }}/{{ FEED_ALL_ATOM }}"` in the template
seems to be the right way to do things, by looking at other themes, eg
the default theme in Pelican itself:

https://github.com/getpelican/pelican/blob/master/pelican/themes/simple/templates/base.html

Obviously, this change might break things for some users of the theme.
If someone used to have this definition in `publishconf.py`:

    FEED_ALL_ATOM = SITEURL + '/feeds/all.atom.xml'

Then they should update to:

    FEED_DOMAIN = SITEURL
    FEED_ALL_ATOM = 'feeds/all.atom.xml'

templates/sidebar.html

@@ -35,7 +35,7 @@
 					</a>
 				{% endif %}
 			{% endfor %}
-			<a class="sidebar-nav-item" href="{{ FEED_ALL_ATOM }}">
+			<a class="sidebar-nav-item" href="{{ FEED_DOMAIN }}/{{ FEED_ALL_ATOM }}">
 				<i class="fas fa-rss"></i>
 			</a>
 		</nav>