R1:d1bcdaeda467

Allow the "Create Subtask" workflow to prompt for a subtype selection, and prepare for customizable options

Summary:
Ref T13222. Ref T12588. See PHI683. Currently, "Create Subtask" always uses the first edit form that the user has access to for the same task subtype. (For example, if you "Create Subtask" from a "Bug", you get the first edit form for "Bugs".)

I didn't want to go too crazy with the initial subtype implementation, but it seems like we're generally on firm ground and it's working fairly well…