Fix showing background img with non-empty baseurl (#497)

* Fix showing background img with non-empty baseurl

Also switch globally to relative_url for all baseurl prepends,
which is more robust than custom URL mangling

* More site.baseurl -> relative_url conversion

_includes/footer.html

@@ -8,7 +8,7 @@
             {%- assign element = site.data.SocialNetworks[curkey] -%}
             <li>
             {%- if curkey == 'rss' -%}
-              <a href="{{ '/feed.xml' | prepend: site.baseurl }}" title="{{ element.name }}">
+              <a href="{{ '/feed.xml' | relative_url }}" title="{{ element.name }}">
             {%- elsif curkey == 'yelp' -%}
               <a href="https://{{ site.social-network-links[curkey] }}.yelp.com" title="{{ element.name }}">
             {%- else -%}