Security update for python-Django (moderate)

This update for python-Django to 1.11.15 fixes the following issues:

This security issue was fixed:

• CVE-2018-14574: Prevent open redirect in
  django.middleware.common.CommonMiddleware (bsc#1102680).

These non-security issues were fixed:

• Fixed WKBWriter.write() and write_hex() for empty polygons on GEOS 3.6.1+
• Fixed a regression that could result in large memory usage when making
  edits using ModelAdmin.list_editable
• Fixed a regression where QuerySet.values() or values_list() after
  combining an annotated and unannotated queryset with union(),
  difference(), or intersection() crashed due to mismatching columns
• Fixed crashes in django.contrib.admindocs when a view is a callable
  object, such as django.contrib.syndication.views.Feed
• Fixed a regression where altering a field with a unique constraint may
  drop and rebuild more foreign keys than necessary
• Fixed a regression where combining two annotated values_list() querysets
  with union(), difference(), or intersection() crashed due to mismatching
  columns
• Fixed a regression where an empty choice could be initially selected for
  the SelectMultiple and CheckboxSelectMultiple widgets