R1:9a10413dbc88

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…