IEP check within schema

[katconnors]

-check inside the schema

Originally posted by @KCCPMG in #335 (review)

[KCCPMG]

Add a check to the IEP table schema in initial_migrations to make sure that the end_date is always greater than or equal to the start_date. This issue should include two quick tests on the student router in student.test.ts, one on addIep and one on editIep, both to ensure that a an iep cannot be submitted when an end date is earlier than a start date, and that the correct postgres error is thrown if this is attempted.