Fix #258 by updated the field app to use "/field/data" for requests

* 32d99ec introduced explicitly named pages to MobileAppDataServlet,
  breaking the behavior of endpoint /field/data/classlist and
  /field/data/upload
* Use /field/data as the endpoint in both cases, as it is supported by
  the index page of the new MobileAppDataServlet
* Update appcache appropriately