Implement sorting in the Solr Search UI

The sorting UI is currently available but not working/partially implemented.

We need to be able to sort results based on Relevance, Title, Date and Last Author.

Certain index/schema changes will also be required because sorting is only possible on indexed, non-tokenized (analyzed) and non multi-valued fields. This means that we will have to provide "<field>_exact" versions of sortable fields that will be of type 'string' and will not be stored, just indexed.

Also, sorting based on author uncovered the requirement of indexing the author/creator's display versions, not only their user IDs, since that is what the user will see and that is what he will expect to be used in the sorting.