DS-1227: Tag cloud for any browse index or Discovery facet

Pull request for the new "Tag Cloud" feature based on Discovery.

Main features:
- Tag cloud can appear in home page, in a community or collection home page and in a signle browse index page.
- Fully parametrized via the Discovery spring XML configuration file and dspace.cfg file
- Enable/Disable feature
- Different style per collection or community

Related JIRA issue: https://jira.duraspace.org/browse/DS-1227

Demo:
Main site: http://esyp.saas.ekt.gr/esyp/?locale=en
Browsing: http://esyp.saas.ekt.gr/esyp/browse?type=subject&locale=en
Community home: http://esyp.saas.ekt.gr/esyp/handle/123456789/5?locale=en

Documentation will be attached shortly!