Werk #16640: Ruleset API: Improve custom validation

To better support the combination of different validation functions, FormSpecs now expect a sequence of validation functions instead of just one for custom_validate.

The validation of empty inputs is now handled in the new validator LengthInRange instead of DisallowEmpty. If you used

custom_validate=DisallowEmpty()

before, use

custom_validate=LengthInRange(min_value=1)

now. For consistency, InRange is renamed to NumberInRange

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