Search: Expand and improve highlighting and excerpting of results

Currently we display term matches from three sources: title, content, and comments.

This issue captures a number of ideas for how to improve the display of our search results and the excerpts that we show.

The issue names no files, tests, or entry points. Start by locating the search-result rendering and excerpting logic, then clarify which highlighting and excerpt improvements are required; the work is done only when those agreed display changes are implemented and verified.

Written by the indexing model from the issue text.