Bump django from 1.11.2 to 2.2.28

Bumps [django](https://github.com/django/django) from 1.11.2 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.2...2.2.28)

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

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

Author: dependabot[bot]

requirements_base.txt

@@ -9,7 +9,7 @@ contextlib2==0.5.4
 coverage==4.2
 daphne==0.14.3
 dj-database-url==0.4.1
-Django==1.11.2
+Django==2.2.28
 django-idmap==1.0.3
 django-redis==4.9.0
 django-vanilla-views==1.0.4