Standardmäßig gecheckte Checkboxes unchecken

[MHajoha]

Siehe FIXME in https://github.com/questionpy-org/moodle-qtype_questionpy/pull/143/files#diff-948b50407bfe3b8d1dc2061e3e7c45d931b700266df8cb7243d356d7abdf0abfR294

// FIXME: Unchecked checkboxes send nothing, so we have no way of distinguishing an explicitly
// unchecked checkbox from a checkbox which was not submitted (e.g. because it wasn't shown).
// As it stands, a default-checked but explicitly unchecked checkbox will be checked again on next
// view.