calendar selenium fixes for rails 3

* fix calendar_event scope (having a 'locked' scope
interferes in query_cache)
* fix user_json when @context is nil

Change-Id: Ic5ac9dc060a13bbc5db02cb3199ceea7598abf5e
Reviewed-on: https://gerrit.instructure.com/30389
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>