Consolidate namespace policy for top-level model selection

The following discussion from !191290 (merged) should be addressed:

• @julie_huang started a discussion: (+1 comment)

  @jpcyiza I modelled these checks off your work here (though I'm also checking current_user is the group owner, which I think is missing from that MR).

  Perhaps later we should refactor to consolidate the logic into a policy, similar to what we do with SHM?