Add index for patchwork_comment (submission_id,date)

Add index for patchwork_comment (submission_id,date)

This (at least theoretically) should speed up displaying comments
on patches/cover letters. It's an index that will return rows
in-order for the query that we always do ("give me the comments
on this submission in date order"), rather than having to have
the database server do a sort for us.

I haven't been able to benchmark something locally that shows
this is an actual improvement, but I don't have as large data
set as various production instances. The query plan does look
a bit nicer though. Although the benefit of index maintenance
versus how long it takes to sort things is a good question.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
[stephenfin: Regenerate migrations per addition of 0027 in earlier
 patch]
Signed-off-by: Stephen Finucane <stephen@that.guru>