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?