Add preference for SDM master key

aleksis/apps/kort/preferences.py

+from binascii import hexlify
+
+from django.utils.translation import gettext as _
+
+from dynamic_preferences.preferences import Section
+from dynamic_preferences.types import StringPreference
+
+from aleksis.core.registries import site_preferences_registry
+
+kort = Section("kort", verbose_name=_("Student ID Cards"))
+
+
+@site_preferences_registry.register
+class SdmMasterKey(StringPreference):
+    """SDM master key for NFC."""
+
+    section = kort
+    name = "sdm_master_key"
+    default = hexlify(bytes(16))
+    verbose_name = _("SDM Master Key")
+    required = False