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 and could probably be shared, but I'd like to wait a bit longer before merging it. For example, both configs have exactly the same structure right now, but Projects should possibly have some different flags (for example: to disable creating subprojects / milestones).

This implementation is pretty basic for now: notably, subprojects/milestones don't get the nice "choose from among subtype forms" treatment that tasks do. If this ends up being part of a solution to PHI1039, I'd plan to fill that in later on.

Test Plan: Defined multiple subtypes, created subtype forms, created projects with appropriate subtypes. Filtered them by subtype. Saw subtype information on list/detail views.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13242

Differential Revision: https://secure.phabricator.com/D20040