Contact Form: Treat checkbox-multiple values as an array.

Since the checkbox-multiple field type should support multiple values, each input field is defined
as part of an array. Later on, the value is imploded into a comma-separated string that is suitable
for displaying to users in the notification e-mail and Feedback post summary.

Prior to introducing the checkbox-multiple field, all fields could be treated as strings, so a
a few sanitzation pieces needed to be updated to also handle arrays.