Fixed hard-coded UI messages.
    
    Some hard-coded messages in ipa.js have been moved into internal.py.
    The messages in internal.py have been rearranged to match the output
    (ipa_init.json).
    
    A new method IPA.get_message() has been added to take a message ID and
    return the translated message or a default message if not found.
    
    Ticket #1701