urls: Add missing path converters for REST APIs

urls: Add missing path converters for REST APIs

Almost all of the API endpoints expect numerical resource IDs, with
'/projects' being the sole exception. However, we were not actually
enforcing this anywhere. Instead, we were relying on the custom
'get_object_or_404' implementation used by 'GenericAPIView.retrieve' via
'GenericAPIView.get_object'. Unfortunately we weren't using this
everywhere, most notably in our handler for 'GET /patches/{id}/checks'.
The end result was a HTTP 500 due to a ValueError.

Resolve this by adding the path converters for all REST API paths in
'patchwork.urls', along with tests to prevent regressions going forward.
We also switch to the DRF variant of 'get_object_or_404' in some places
to provide additional protection.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Cc: Daniel Axtens <dja@axtens.net>
[dja: s/TypeError/ValueError/; dropped an unrelated change]
Signed-off-by: Daniel Axtens <dja@axtens.net>