optional = true does not make sense without a datatype. When no datatype is
set, the empty string will be a valid value, so data is never unset in the
write function. Restore the minlength(1) datatype so the contact setting is
deleted as intended when no value is provided.

gluon-config-mode-contact-info: provide enhancements for german, english and french translation to comply with DSGVO (#1394)

* do not allow to obligatorily require contact information
* add remark that the data is provided voluntarily
* mention how to delete the data
* be very clear about the fact that the data being entered is public and
  can be downloaded and processed by anyone.

Also add documentation about config mode settings.

simple-uci will now delete an option when nil is passed.

We also make use of the boolean support of simple-uci to make scripts
clearer.