Attendance follow-up

[etrepum]

These could be broken down into separate issues/PRs, collecting into one issue as that's more convenient to do for the review of #15. I'll be editing this issue as I go along to collect the feedback.

attendance.views.attendance

• Presumably these views should require staff level authorization, rather than just a login
• Input validation is missing
• The POST handling code (in the success case) could be simplified using Post/Redirect/Get
• Date parsing should be factored out to a function, possibly made more robust (as in More robust birthdate parsing #13)

attendance.views.compile_attendance_averages_for_all_courses

• This is likely to be a bottleneck, should be computed out of band from the request (maybe with a trigger of some kind on update, and/or updated periodically) and then cached. Salesforce probably has a mechanism for this with Reports, but there are of course other ways to offer this functionality. The manner in which this is computed is also likely to be quite slow, as it fetches all of the data over the course of several queries and then aggregates it from Python rather than doing this work at the database.