R1:1ac52c09e757

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.