[DS-1912] Discovery facets do not always use the authority identifier

The code currently always uses the displayedValue as a filter parameter, but this is not always correct. Due to the following change in the backend: https://github.com/DSpace/DSpace/blob/master/dspace-api/src/main/java/org/dspace/discovery/SolrServiceImpl.java#L1820-1825 . The displayedValue is sometimes overwritten by the authority (if present) so the UI should also takes this into account.

You can test this pull request by setting a sidebar facet to an authority controlled metadata field and selecting a field that has an authority linked to it (but with a similar displayed value in a non authority related field).