Bump django from 2.2.19 to 2.2.28 in /requirements

Bumps [django](https://github.com/django/django) from 2.2.19 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.19...2.2.28)

---
updated-dependencies:
- dependency-name: django
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/base.txt

@@ -1,5 +1,5 @@
 celery==4.4.6 # pyup: <5.0
-Django==2.2.19 # pyup: <3.0
+Django==2.2.28 # pyup: <3.0
 django-cleanup==5.1.0
 django_compressor==2.4
 django-modeltranslation==0.16.1