Validation for duplicate breakfast_options

[peter-ohara]

More than one breakfast_option can be created with the same meal_id and serving_date.
Write model validations to prevent this from happening.

In addition, create a unique index on the breakfast_options database table to enforce this rule, even when the model validations are bypassed. (Model validations can be bypassed when there is a race condition, when that happens we expect that the database will enforce the uniqueness index and prevent duplicates as needed)