Add a toggle to show one's telephone number to all members

The default is (obviously) set to false. But now members can choose to
make their telephone number public.
Use preferences.visibility.telephone, not telephoneVisible

Also make a new API call for preferences.visibility, which never truly
fitted well in entity_update_primary.
Add forgotten </dt>
Merge pull request #242 from aykevl93/telephone-number-toggle

Maak telefoonnummers optioneel zichtbaar voor leden