implement password change for mailboxes

- implement managemails.forms.ChangeMailboxPasswordForm
- extract code for determining hosting package and customer from URL into
  HostingPackageAndCustomerMixin
- implement managemails.views.ChangeMailboxPassword
- add new URL pattern 'change_mailbox_password' to managemails.urls
- add template managemails/mailbox_setpassword.html
- link from template hostingpackages/customerhostingpackage_detail.html to
  change_mailbox_password
- add german translation for new strings
- document new feature in changelog

gnuviechadmin/templates/managemails/mailbox_setpassword.html

@@ -0,0 +1,34 @@
+{% extends "managemails/base.html" %}
+{% load i18n crispy_forms_tags %}
+{% block title %}{{ block.user }} - {% spaceless %}
+{% with full_name=customer.get_full_name mailbox=mailbox.username %}
+{% if customer == user %}
+{% blocktrans %}Set Password for Mailbox {{ mailbox }}{% endblocktrans %}
+{% else %}
+{% blocktrans %}Set Password for Mailbox {{ mailbox }} of Customer {{ full_name }}{% endblocktrans %}
+{% endif %}
+{% endwith %}
+{% endspaceless %}{% endblock title %}
+{% block page_title %}{% spaceless %}
+{% with full_name=customer.get_full_name mailbox=mailbox.username %}
+{% if customer == user %}
+{% blocktrans %}Set Password for Mailbox {{ mailbox }}{% endblocktrans %}
+{% else %}
+{% blocktrans %}Set Password for Mailbox {{ mailbox }} of Customer {{ full_name }}{% endblocktrans %}
+{% endif %}
+{% endwith %}
+{% endspaceless %}{% endblock page_title %}
+
+{% block content %}
+<p>{% if customer == user %}{% trans "Please specify the new password for your mailbox." %}{% else %}{% trans "Please specify the new password for the mailbox." %}{% endif %}</p>
+{% crispy form %}
+{% endblock content %}
+
+{% block extra_js %}
+<script type="text/javascript">
+$(document).ready(function() {
+  $('input[type=password]').val('');
+  $('input[type=password]').first().focus();
+});
+</script>
+{% endblock extra_js %}