preliminary sorting logic
require development version of eulexistdb for now, to support sorting results on custom xpath [#60340184]
logic to sort results by title, relevance, or date, with custom xpath for sort date [#60340184]
display links to re-sort current result set by any of the sort optiosn [#60340184]
Merge branch 'feature/sort' into develop

Conflicts:
	ddisearch/ddi/views.py