Represent deadlines / expiration dates as datetimes

[jennifer-richards]

Description

As a moderate complexity but wide scoped refactoring, places where dates are used to represent deadlines / expiration dates and the like should be replaced with datetimes. Using a bare date means the timezone is ambiguous and relies on keeping the database and code synchronized.

Replacing the date with midnight in the intended timezone should have the same effect, so migration should be straightforward.

This includes scraping the code for places where we're using date_today, replacing DateField with DateTimeField, updating code and form handling where these dates are set, and adjusting how they are displayed and used in comparisons.

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