Bump django from 2.2.15 to 2.2.28

Bumps [django](https://github.com/django/django) from 2.2.15 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.15...2.2.28)

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

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

requirements.txt

@@ -1,4 +1,4 @@
-Django==2.0.13
+Django==2.2.28
 psycopg2-binary==2.8.5
 django-easy-select2==1.5.7
 Markdown==3.0.1