Custom field validation

[mjashanks]

Currently, we can add some simple validation rules against a row - such as Required, Max and Min.

I propose that we should have the ability to write some more custom validation.

Such as

• Text field A is Required if Checkbox field B is checked
• End Date must be after Start Date
• Estimated Value is required if Logged in User.Role is Finance

I feel like the best option would be to declare these on Forms, so that they could be used by both internal and external datasources.

Therefore, feels related to #1617

[mjashanks]

Also, would be good to include some extra predefined validations

• email
• URL

Im sure there are others.