REST: Correct some prefetch, select_related

REST: Correct some prefetch, select_related

There were two issues here:

- The 'get_queryset' function, rather than the 'queryset' attribute,
  must be overriden when using either the 'prefetch_related' or
  'select_related' functions

- A couple of endpoints contained a 'project' attribute, but this wasn't
  being prefetched. This didn't cause issues in a single-project
  deployment, as used in testing, but will for larger deployments

Resolve both issues.

Signed-off-by: Stephen Finucane <stephen@that.guru>