Team splitting doesn't work

[blerner]

We have a schema requirement that InlineComments have non-nil grade fields. When we split a team, we create a comment to acknowledge that, with a grade_id = 0, which corresponds to a nonexistent Grade...but it's technically not nil and so passes the database schema. We can make the belongs_to :grade be optional: true in InlineComment, which bypasses this check...but we might consider also making grade be nilable after all...

[blerner]

I suspect this will affect Mark as Plagiarized as well.