Add [Required] attribute to DB properties

[morganbr]

If a property on an Entity Framework table is a nullable type (such as String or another class), the column is treated as optional and the DB will accept null. For values that shouldn't be null, we should set the [Required] attribute.

[morganbr]

This will also cascade deletes to related entities, which we mostly want. For example, Setting it to the Event property of Puzzle will make it so deleting an event automatically deletes its puzzles.