R1:c5b13a6be3b7

Allow object subtypes to be changed via bulk editor

Summary:
Ref T13151. See PHI683. Ref T12314.

You can currently change object subtypes via Conduit (`maniphest.edit`) but not via the web UI.

Changing object subtypes is inherently a somewhat-perilous operation that likely has a lot of rough edges we'll need to smooth over eventually, mostly around changing an object from subtype X to subtype Y, where some field exists on one but not the other. This isn't a huge issue, just not entirely intuitive.

It…