Extra validations for selectboxes, select and radio

[sergei-maertens]

Context: #53 (comment)

Currently you can add manual options/values with:

• duplicated values, which is confusing when mapping back from values to labels (confirmation, summary...)
• spaces, periods or other special characters in the values, which is normally okay but makes lookups by value odd (like selectboxes' weird datastructure would probably result in a nested object instead of using the literal key with period in it) and also notably makes processing those values in the Django template engine hard in the backend.

It is verified that Formio itself in its own builder does not validate these things, so if we change the behaviour, then it will be an additional constraint. We will also have to check for existing form definitions that would suddenly become invalid and prevent updating/saving the form again, so we need to be careful about this.

[sergei-maertens]

open-formulieren/open-forms#3668 -> the value must be able to take on any value, it should not be slug validated.

behaviour is now locked in with a test: b01c0e1

[sergei-maertens]

Not worth the effort at this stage.