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.
Do not indent subscription description
Fix <h1> margin

There wasn't any margin underneath. Fix it so there is some.
Fix commit 75db7431: don't use <p> around generated HTML

Instead, use a proper <div>.
Provide proper fallback for old subscriptions

Use django.utils.html.linebreaks to automatically add <p> and <br/>
tags.
Merge pull request #243 from aykevl93/subscriptions-layout

Activiteiten layout
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