Use existing todos conventions for min/max constraints

[pkalita-lbl]

These changes update the Validator class to be consistent with existing conventions around storing minimum and maximum date values in todos with the >= and <= prefixes. It also allows enables the use of the convention of >={other slot name} or <={other slot name} in todos to indicate that the min/max value for one slot is dependent on the value of another slot.

I added cases to the unit test and did some basic interactive testing with the covid template. I think it's working as expected without any changes to the existing schema. But please give it a good test and let me know if you find any existing schemas that aren't being interpreted correctly!