Cleanup Warnings & Fix Bugs

- Address deprecated warnings (use setFilter with FilterPredicate rather
than addFilter)
- Address unavoidable unchecked cast warnings (change setting in
Eclipse)
- Annotate anonymous score functionality in School as deprecated
- Escape content on error pages for HTML
- Make registration form not validate password if not creating account
but password is nonempty
- Send 400 when invalid level parameter specified instead of 500 (from
IllegalArgumentException)