Add very basic UI for creating milestones and subprojects

Summary:
Ref T10010. This has a lot of UI/UX problems but I think it:

• technically allows subproject creation;
• technically allows milestone creation;
• doesn't let users unwittingly destroy their installs (probably).

Test Plan:

• Created milestones.
• Created subprojects.
• Created and edited normal projects.
• Observed some reasonable interactions (e.g., you can't create milestones for a milestone or edit a superproject's members).
• Observed plenty of silly/confusing interactions that need additional work.

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Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14904