Set timestamp columns to be NOT NULL where we depend on their values

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Rails manages created_at and updated_at columns in tables for us automatically, most of the time, but on occasion - as with gitlab-ce#20568 - rows may be created with NULL values. This breaks a lot of code paths that expect these columns to always have values.

I propose that we change these columns to be NOT NULL, since we depend so strongly on them being so in our code. Input validation is great!