rapidftr/tracker#65 ordering potential matches by relevance.

@mwangiann @ctumwebaze @andreweskeclarke 

This fix adds;
1. An interface for the administrator to define a score threshold below which solr results will be rejected.
2. Removes potential matches that the user has not interacted with (confirmed/marked as invalid) and recreates them
 when a solr match is done.
3. Changed the potential matches search to return a hash of the model identifiers with their scores.
4. Returning potential match objects instead of children from Enquiry#potential_matches, in order to show the score
5. Displays the score along with the child in the summary panel of the enquiry's matches.