Improve typeahead behavior with mixed-case matches

Summary:
Ref T8510. We had two issues with mixed-case result sorting, like typing @joe to match user Joe.

• The fallback sort was not normalized properly, so "J" could sort after "j". Instead, normalize values for sorting.
• The prefix_hits and older priority_hits mechanisms were competing destructively. The prefix_hits mechanism completely replaces the priority_hits mechanism. Instead, use only the prefix_hits mechanism.

Test Plan:

• Copied results for "joe" from WMF.
• Hard-coded the controller to return them.
• Searched for @joe.
• After patches, first hit is user "Joe".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8510

Differential Revision: https://secure.phabricator.com/D16826