DS-2108 Add an XMLUI aspect to report Google Analytics stats

http://jira.duraspace.org/browse/DS-2108

A couple of minor issues:-

1. I created a separate messages.xml file but this could be merged into main one.
2. There might be another commit required to edit the sitemap's for the Solr and ElasticSearch aspects, but I have left it out for the moment so as not to pollute this PR.

Note:- this feature is currently dependent on the XMLUI Mirage2 theme, anyone fancy developing for other themes or for the JSPUI?
There seem to be new versions of these dependencies on Maven Central. I'm a little worried whether the "-rc"s will stay there forever.