Add fallback search for case where Haystack isn't installed

Fix #268.

If anyone has a better fallback search strategy in mind or has tests to propose please comment on this PR and I will update before this gets merged in.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Is `icontains=content` kosher?  Maybe the fallback should just be to always return an empty queryset; otherwise, don't we have to search a few different fields in DataField, DataConcept, and DataCategory for the content (completely ignoring the data itself, for sanity)?  Is the motivation of the parent issue that Haystack requires old Django?  In this case, maybe we should also think about helping to update Haystack ....