AppointmentGroup.context returns first AppointmentGroupContext.context

accessor was hiding a method. looks like we want to use the method
because of multi-context appointment groups

test plan
- from the console, ensure that a new appointment group returns
 non-nil from .context

Change-Id: I36ef6bc073b4e1d0bcf02277b1e4fdec43630382
Reviewed-on: https://gerrit.instructure.com/32254
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>