Improve backend lookup in additional results block and zero results page.

* Don't show "Remove limit(s)" link unless there is a query (so we don't lead to an all results page).
* Don't show the additional results block on any page but the first page.
* Don't execute backend lookup until the link is in the viewport to minimize solr queries.
* Removed disabled Article search link.
* Only link to WorldCat/Library search when there is a query to search.
Merge pull request #706 from sul-dlss/diff-results

Improve backend lookup in additional results block and zero results page