Checkboxes can be saved as "multiple" or non-multiple

[brunobord]

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In the context of the builder, if you add a "checkboxes" field, it appears that the front-end may or may not send us the "multiple" property. If not, when we save the form, the field.multiple value is the default one, which is False.

we could override this by forcing the front to send us a "multiple" value OR, if missing, default it to "True" in the case of a "checkboxes" field.