Will/fix preview errors

Fixes for three errors that occur when viewing the XBlock in Studio preview:

1) `DoesNotExist` error caused by looking up `StudentItem` with `student_id` set to `None`.
2) `ValueError` caused by trying to parse timestamp with milliseconds (which is the default value set by `isoformat`)
3) AJAX requests in `oa_base.js` were interpreting the response as JSON and registering an AJAX error when  HTML was found instead (syntax error about "<" chars).  Studio catches these errors and assumes that an error occurred while saving, so it shows a (very misleading) error message.

@ormsbee