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.
@christianv , I just got a chance to look at this PR, sorry. This has got me very worried. get_enrolled_students gets called in bulk from a bunch of different places and needs to be fast and efficient. Getting student photos is *not* fast and efficient, and is *not* needed by a bunch of different places. I would really recommend keeping the photo queries separated out as much as possible.