Unified Ajax script handler

Create a core ajax script handler (and AMD wrapper) with these features:

• Can call any registered webservice using sesskey as a token
• Can call multiple functions in one request and return separate responses
• Can register callbacks to receive the response from:
  • Success of all requests
  • Any single failure
  • Success or failure of any specific request

It will be a big improvement if we can stop building separate AJAX apis and web service APIs, as well as many separate AJAX handlers with no consistent responses/error handling.