Enforce determinism in contest ratings

[xiaowuc1]

Contest ratings evaluate sorting on end time of contests. To ensure determinism, end times should be unique and this should be enforced at the database level.

[xiaowuc1]

In a perfect world, rated contest windows would also be disjoint. This is harder to enforce at the DB level.

[int-y1]

• add unique=True to the end_time field
• add a migration that makes end_time unique, and then updates the field