[#6610] Improve trac ticket content formatting

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6610] Don't replace shortlink patterns in raw text blocks

- This was necessary when we were fetching raw ticket text during export.
  Now that we're fetching html, things that should be links are already
  <a> tags. Any remaining shortlink-like patterns in the ticket text
  should *not* be converted to links.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>