CLC-4064 - Roster Photos Feature doesn't print nicely

https://jira.ets.berkeley.edu/jira/browse/CLC-4064

When there is no picture available, the API shouldn't return any. On the front-end we check for it and if it doesn't exist, we show the no photo available.
Whoa, big N+1 query performance problem here: This runs the get_photo query once per student in each class. Better by far to join the photo column into the get_enrolled_students query. Let me know if you need help on that.