[kjlippold]

Addresses #560

1. Disables the default admin login page. /admin will now direct users to /accounts/login/ and redirect back to the admin dashboard after logging in.
2. Non-admin users who try to access admin pages will be redirected to their profile page with a notification that they don't have permission to access the admin dashboard.
3. Added a link to the admin dashboard from the data management app, only visible to admin users.
4. Removed the computed "accountType" field. All callers of that field now use the isSuperuser, isStaff, and ownedWorkspaceLimit fields instead.