chore(deps): bump django from 2.2.9 to 2.2.18 in /requirements

Bumps [django](https://github.com/django/django) from 2.2.9 to 2.2.18.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.9...2.2.18)

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

requirements/base.txt

@@ -18,7 +18,7 @@ whitenoise==4.1.4
 
 # Inherited a4-core requirements
 bleach==3.1.0
-Django==2.2.9 # pyup: <2.3
+Django==2.2.18 # pyup: <2.3
 django-allauth==0.40.0
 django-autoslug==1.9.6
 django-background-tasks==1.2.0