Bump django from 2.2.4 to 2.2.8 in /cvat/requirements (#902)

Bumps [django](https://github.com/django/django) from 2.2.4 to 2.2.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2.4...2.2.8)

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

cvat/requirements/base.txt

@@ -1,5 +1,5 @@
 click==6.7
-Django==2.2.4
+Django==2.2.8
 django-appconf==1.0.2
 django-auth-ldap==1.4.0
 django-cacheops==4.0.6