R1:2f11001f6e3c

Allow "Change Subtype" to be selected from the comment action stack

Summary:
Ref T13222. See PHI683. Currently, you can "Change subtype..." via Conduit and the bulk editor, but not via the comment action stack or edit forms.

In PHI683 an install is doing this often enough that they'd like it to become a first-class action. I've generally been cautious about pushing this action to become a first-class action (there are some inevitable rough edges and I don't want to add too much complexity if there isn't…