Bump django from 2.2.24 to 2.2.28

Bumps [django](https://github.com/django/django) from 2.2.24 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.24...2.2.28)

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

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
-Django==2.2.24
+Django==2.2.28
 django-widget-tweaks==1.4.2
 selenium==3.14.0
 pytz==2018.5