Turn off subprojects and milestones in the UI completely for now

Summary: A lot of confusing stuff happens when you create a milestone or subproject which isn't explained clearly in the UI. This is causing more harm than good on the balance since we're still figuring out how to move forward here. Just turn it off for now until we're closer to pushing it forward.

Test Plan: Viewed a project, no more UI for subprojects or milestones.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14999