Disable "Subprojects" menu item for milestone projects

Summary:
Ref T10010. Milestones can't have subprojects, so this item isn't very useful.

I think there is also an argument for disabling "Members", but that panel is a little less useless and explains the membership rule, so I'm less certain about removing it. I do generally lean toward removing it at some point, though.

Test Plan:

• Viewed a milestone, no "Subprojects" menu item.
• Viewed a normal project, saw item.
• Edited both menus, saw consistent UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15200