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/navbarlink.html

@@ -8,7 +8,7 @@
 {% endif %}
 
 {% if internal %}
-  {% capture linkurl %}{{ site.baseurl }}/{{ include.link[1] }}{% endcapture %}
+  {% capture linkurl %}{{ include.link[1] | relative_url }}{% endcapture %}
 {% else %}
   {% capture linkurl %}{{ include.link[1] }}{% endcapture %}
 {% endif %}