R1:c9760e8d643b

Support subtypes in Projects

Summary:
Ref T13242. See PHI1039. Maniphest subtypes generally seem to be working well. I designed them as a general capability that might be extended to other `EditEngine` objects later, and PHI1039 describes a situation where extending subtypes to projects would give us some reasonable tools.

(Some installs also already use icons/colors as a sort of lightweight version of subtypes, so I believe this is generally useful capability.)

Some of this is a little bit copy-pasted…