[DS-1272] Enable Discovery By Default for the XMLUI

This pull request will enable discovery by default.

Some remarks about the pull request:

* The discovery solr impl has been altered so that if no valid solr url is provided it will not start throwing exceptions, it will graciously skip all discovery methods (this is the case for the api tests)
* I also moved the "browse" links out of the CommunityViewer & CollectionViewer classes, so if you turn off the browse aspect all the browse links are now gone from the user interface (as one would expect)

Work not included in this pull request:
* The browse aspect is still enabled (since this serves a different use case than discovery)
* Enabling discovery for the jspui
* Adding update-discovery-index to index-init & index-update, although if this requested I will alter it