Bump django from 2.2.8 to 2.2.28 in /config

Bumps [django](https://github.com/django/django) from 2.2.8 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.8...2.2.28)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

config/requirements.txt

@@ -13,7 +13,7 @@ django-debug-toolbar==3.1.1
 django-localflavor==1.0
 django-markdownx==2.0.27
 django-picklefield==3.0.1
-django==2.2.8
+django==2.2.28
 easy-thumbnails==2.7.1
 fabric==2.4.0
 gunicorn==19.9.0