IDV is on its way to retirement, so it's not going to be necessary for cert
generation forever.
Introduces a function to combine the honor code flag with IDV to tell
cert generation if it should care about a missing verification.
Various tests expanded to cover the retired case. The additional calls
in test_task_helper.py are caused by one call to fetch course
overrides which finds none, and that forces one check of the
background flag per student, 71 + 1 + 5 = 77.
MST-854
7e3ef93a63