Search engine

Please check whether you agree with the "search" pull request **first**.  
Also, a check/discussion on what should be visible is welcome as well

This pull request enhances the search branch, by adding Solr as search engine.
Also, **Solr has to be installed before deploying** 
(development and test use a gem, but production should ideally use a standalone version; add the gem to production if we wish to use a search engine without installing Solr just yet...)

Docs still have to be written :sob: