Add covering index to patchwork_submissions for /list/ queries

Add covering index to patchwork_submissions for /list/ queries

This gets PostgreSQL to generate *much* better query plans, gaining us
about two orders of magnitude in performance on the /list/ query for the
worst state project on the patchwork.ozlabs.org instance (qemu-devel).

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>