adjust cohort factories to have a start_date on monday and end_date on friday

Running the tests near or on the weekend caused cohort related specs to fail.
This was because the start date of the factory cohort was dependent on the
current date. Now it's still based on the current date, but always falls to
the monday (for start_date) or friday (for end_date) of the week.
adjust today scope and validation to use utc time.

The way we validated the uniqueness of attendance records was by
validating a unique user_id scoped to Date.today. Unfortunately,
this didn't work quite as intended. Postgres saves dates in UTC,
so even though the system knows to use Pacific Time, querying
with 'DATE(created_at)' pulled those records that were created_at
the utc time, and therefore didnt' properly match Date.today (which
is in Pacific Time).
add specs for attendance_record model
refactor attendance_record uniqueness check
Merge pull request #23 from epicodus/time-adjustments

Time adjustments