Improves filtering of Records

Adds a Record.filter() classmethod as a wrapper around
Record.objects.filter(). In addition to filtering by all the fields of
the Record model, it also accepts an optional bool parameter only_today,
which if True filters for Records only from the given day, which can be
useful for applications such as teacher check-in.

Modifies Record.user_completed() to also accept the only_today
parameter, and to use Record.filter() internally rather than construct
its own QuerySet.
Modifies teacher check-in to grant Records by day

Uses the new/improved Record.filter() and Record.user_completed()
methods to check for existing Records only for the given day.

Partially resolves Github issue #818.
Fixes multi-day teacher check-in

Only checkins from the date of 'when' (which defaults to the current
date) are considered, so that check-in can be reused each day/week of
the program. To see who failed to check in on a certain day / by a
certain time, admins can use the pre-existing "previous day" feature.

Resolves Github issue #818.
Sorts teacher check-in by time of class
Adds RecordTest class
Adds TeacherCheckinModuleTest

Starts by writing runCheckInTest(), which tests checkIn() and
undoCheckIn().

More tests forthcoming.
Merge pull request #1209 from learning-unlimited/issue-818

Issue 818