Refactor / simplify search results.

* Split list.html template into two:
  list.html for rendering the plain list with no query,
  search.html for rending query results.

  This is nicer than having a bunch of logic in the template.
  And there isn't that much duplication, although probably this
  could be improved further.

* Instead of QueryResult having a benchmarks field and
  matches_by_benchmark field which is a dict keyed by benchmark name,
  just have a matches field which is a list of (benchmark, num_matches)
  tuples.