feat(app-auth): Improve client-side Auth service

Changes:
- getCurrentUser, isLoggedIn, and isAdmin are now sync if no arg and async with an arg
- Use Error first callback signature where applicable
- Remove unused arguments from Auth service
- Remove isLoggedInAsync
- Switch use of isLoggedInAsync to isLoggedIn
- Add/Improve comments
- Fix client/app/account(auth)/settings/settings.controller(js).js

Breaking Changes:
- Callbacks that return Errors, use 'Error first' signature

Closes #456