Fix next/prev scrolling for author facet searches

When running a keyword search, then choosing an author hyperlink from the
results to run an author facet search, the next/previous links in the individual
results would be based on the initial keyword search, not the facet search.

This was for two reasons:
- the ResultScroller was not active in the AuthorController, which handles
author facet searches
- the ResultScroller retrieves the current search from the session to create
the scrolling links, and author facet searches were configured to not
be saved in the session

Correct these two issues in AuthorController, but only if next_prev_navigation
is enabled. Because AuthorController also handles SolrAuthorFacet searches
(searches that produce a list of authors, not a list of titles by a specific author)
and those results cannot be iterated through by a ResultScroller, we also have
to make an additional fix in the ResultScroller plugin to handle the case
where SolrAuthorFacet result is passed to create the ResultScroller, and
prevent it from being instantiated.