Handle config settings more robustly

[stijn-uva]

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It is now possible to tweak the 4CAT settings through the web interface, but this also makes it easier to break 4CAT by entering an invalid value, or by entering something that confuses the internal parsing.

There should be a generic way to define constraints for config values, e.g. through a lambda function that returns true or false depending on the value, which is defined in the option definition. Additionally, empty values should be handled consistently - at the moment some values use an empty string while others use None, which can also confuse the code.