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/managemails/urls.py

@@ -8,6 +8,7 @@ from __future__ import absolute_import, unicode_literals
 from django.conf.urls import patterns, url
 
 from .views import (
+    ChangeMailboxPassword,
     CreateMailbox,
 )
 
@@ -15,4 +16,6 @@ urlpatterns = patterns(
     '',
     url(r'^(?P<package>\d+)/mailbox/create$',
         CreateMailbox.as_view(), name='create_mailbox'),
+    url(r'^(?P<package>\d+)/mailbox/(?P<slug>[\w0-9]+)/setpassword$',
+        ChangeMailboxPassword.as_view(), name='change_mailbox_password'),
 )