Replace Lucene search for Review Requests with Haystack and Whoosh.

Full-text search with Lucene is a hassle to set up. Haystack and Whoosh are
pure python so it's easy to include them with Review Board and have full-text
search available out of the box. Regular maintenance of the search index is
still required for the search function to return up-to-date results.

These Review Request fields are indexed : id, summary, description, commit_id,
testing done, bugs, author (submitter's full name), submitter (username),
filenames of the files in the diffs (diff content is not indexed).

Testing Done:
* Enabled and disabled search in General Settings and changing index file path
  and results per page, followed by rebuilding the index.
* Search using the following fields:
      id, summary, description, testing done, bugs, author, submitter,
      diff filenames.
* Built and inspected Sphinx docs.
* Built and updated the index through rb-site.
* Unit tests passed.

Reviewed at https://reviews.reviewboard.org/r/4935/

Obsoletes bugs 2021, 2544, 2755, and 3095