Initial stab at rewriter for `IndexedComment`

When we put things into the `IndexedComments` table, they are
transformed to remove common words, normalize urls, remove
capitalization, remove markdown formatting, strip punctuation, and more.

Because the inputs are simplified, we can then perform exact text
matches on rows while handling a reasonable amount of fuzziness. This,
paired with a big enough database should allow us to respond to *some*
comments.