add model features to hostingpackages

- add properties mailboxes, used_mailbox_count, mailbox_count and
  may_add_mailbox to CustomerHostingPackage class
- use new properties in template dashboard/user_dashboard.html

gnuviechadmin/templates/dashboard/user_dashboard.html

@@ -28,7 +28,7 @@
                 <span title="{% blocktrans %}The reserved disk space for your hosting package is {{ diskspace }} bytes.{% endblocktrans %}">{{ diskspace|filesizeformat }}</span>
                 {% endwith %}
               </th>
-              <th>{% blocktrans with num=package.get_used_mailboxes total=package.get_mailboxes %}used {{ num }} of {{ total }}{% endblocktrans %}</th>
+              <th>{% blocktrans with num=package.used_mailbox_count total=package.mailbox_count %}used {{ num }} of {{ total }}{% endblocktrans %}</th>
               <th>{% for dbtype in package.get_databases %}
                 {{ dbtype.number }} {% include "userdbs/snippets/db_type.html" with db_type=dbtype.db_type %}
                 {% if not forloop.last %} / {% endif %}