Improve search highlighting for CJK and substring queries

Summary:
Fixes T12995. Currently, the result highlighter (which shows where terms matched) only works in "term" mode, not in "substring" mode.

Provide better feedback and behvaior:

• When a term is a substring term, color it a little differently and add a tooltip. (This is partly to make it easier to debug/diagnose things, probably not enormously valuable to users.)
• When a term is a substring term, highlight it anywhere in the results.

Test Plan:
Queried for latin and CJK terms.

Here is CJK being highlighted:

Here is substring vs non-substring implicit behavior:

Here's ONLY terms being highlighted:

Here's terms and substrings, since the query now has a substring:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12995

Differential Revision: https://secure.phabricator.com/D18635