Implement list filtering

Implement list filtering

Sometimes, multiple projects reside at the same mailing list. So far,
Patchwork only allowed a single project per mailing list, which made it
impossible for these projects to use Patchwork (unless they did some
dirty hacks).

Add a new property `subject_match` to projects and implement filtering
on (list_id, subject_match) match instead of solely list_id. Instance
admin can specify a regex on a per-project basis when the project is
created.

Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>