Implement `TryFrom<Form>` for all entities

[Shadow53]

• Uses the custom error type (Implement custom error type #2) in the Err case
  • Error messages should clearly indicate the error without leaking information
  • The appropriate status code should be used every time
• Performs rigorous input validation
• Should assume that all of the information should be present; i.e. it is an error if anything required is missing
• Should be feature-gated.
• Unit tests should test both valid Forms and various kinds of invalid ones
• Uses the Form type from the nebula_form crate

TODO: Add checklist items for every entity once the data design is complete