Make "profile menu" configuration mostly work

Summary:
Ref T10054. This does a big chunk of the legwork to let users reconfigure profile menus (currently, just project menus).

This includes:

• Editing builtin items (e.g., you can rename the default items).
• Creating new items (for now, only links are available).

This does not yet include:

• Hiding items.
• Reordering items.
• Lots of fancy types of items (dashboards, etc).
• Any UI changes.
• Documentation (does feature: TODO link for documentation).

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15010