Show an "approval queue" item on the home page for admins, and sort out menu item visibility

Summary:

• If you're an administrator and there are users waiting for approval, show a count on the home page.
• Sort out the isUserActivated() access check.
• Hide all the menu widgets except "Logout" for disabled and unapproved users.
• Add a "Log In" item.
• Add a bunch of unit tests.

Test Plan: Ran unit tests, clicked around as unapproved/approved/logged-in/logged-out users.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Differential Revision: https://secure.phabricator.com/D7574