Overhaul search results style

[srawlins]

This should fix #3171 and #3175. It's a big PR as I refactored a lot of code so that I could understand it better.

Mini results

Full page

• Separate the style of results shown in the "mini results" below the search bar, and those on the full search page.
  • Mini search results below search bar:
    • Use @Hixie's intuition that each result should be a single line of text
    • Add 60 pixels to the width, to accommodate for the single line results
    • Hide text overflow, but each result's full "one line doc" is available as title text (hover)
  • Full search bar: more or less unchanged
• Search code
  • initializeSearch was ~450 lines, so I split out the local functions where I was editing code.
  • Change anything named "category" or "group" to be "container" which more closely aligns with other dartdoc terminology for "a thing that can be an enclosing element".
  • Delete iterateCategoriesMap and just inline its code.
  • Rename handle to handleSearch.
  • Change many comments to doc comments.
  • Wrap many comments.
  • Change handle's bool optional parameter to be named, to fit with Effective Dart.

There is a fair amount that could still be improved here (see other bugs), but (a) we don't have any automated testing for the generated JS app 😦 and (b) I should stop here and land something.

[srawlins]

CC @keertip @kevmoo

[keertip]

Difficult to review due to the refactor, the diff does not really show only code that has changed. Went by your comments as to the changes.

[srawlins]

I know, I apologize ☹️ . I tried to stop refactoring as early as I could. And I didn't want 2 or 4 refactor PRs to precede this one as I hope to get this fixes out quickly. My bad for rushing it maybe. Follow up PRs for search should be smaller.