Upgrade to Django 3.1

- upgrades
  - Django 3.1.14
  - djangorestframework 3.13.1 (from 3.12.1)
  - djangorestframework-yaml 2.0.0 (from 1.0.3)
  - django-polymorphic 3.1.0 (from 2.1.2)
  - social-auth-core 4.2.0 (from 3.3.1)
  - social-auth-app-django 5.0.0 (from 3.1.0)
  - django-auth-ldap 4.0.0 (from 2.1.0)
  - django-crum 0.7.9 (from 0.7.5)

- django-jsonfield is removed

docs/licenses/django-jsonfield.txt

@@ -1,23 +0,0 @@
-Copyright (c) 2012, Matthew Schinckel.
-All rights reserved.
-
-Redistribution and use in source and binary forms, with or without
-modification, are permitted provided that the following conditions are met:
-    * Redistributions of source code must retain the above copyright
-      notice, this list of conditions and the following disclaimer.
-    * Redistributions in binary form must reproduce the above copyright
-      notice, this list of conditions and the following disclaimer in the
-      documentation and/or other materials provided with the distribution.
-    * The names of its contributors may not be used to endorse or promote products
-      derived from this software without specific prior written permission.
-
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
-ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-DISCLAIMED. IN NO EVENT SHALL MATTHEW SCHINCKEL BE LIABLE FOR ANY DIRECT,
-INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
-BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
-LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
-OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
-ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.