Week 12 challenges added
Compile the IDs of Phase 0 challenges without a call to the database
Limit exercises to the 20 required in Prep

Previously, all exercises were included in the database, but students are only required to complete 20 exercises during Prep.  Most of the others have been
converted to challenges that are completed during Phase 0.
Save data for only the most recent attempt of each exercise

Based on user feedback, there was no need to keep a record of each exercise attempt.  Users want to see which exercises have been completed and which have not.  A student's most recent code and submission date is saved for each attempted exercise.  This significantly reduces the number of records in the database.
Only cohorts not in session in database

The original set up recorded all cohorts returned by the DBC API in the database.  This change limits the cohorts recorded to just those which have not started at DBC:  those in Phase 0 and the miscellaneous cohorts (e.g., yearly melt or hold).
Limit users in db to admins and students whose cohorts are not in session

Previously, all users returned by the DBC API were recorded in the database.  The users table grew to more than four times what was necessary and was only going to grow worse as the first cohorts exit Phase 0.
Ensure removal of attempts by students no longer on Phase 0
Limit challenge attempts to one per student

Users were not comparing multiple challenge submissions.  This change keeps one record per challenge attempted--the most recent attempt.
Merge pull request #25 from toreyhickman/seed

Limit data in database to what is currently needed for Phase 0