Upgrade to Wagtail 2.8, Django 3.0

requirements/base.txt

@@ -1,5 +1,5 @@
-Django>=2.2,<2.3
+Django>=3.0,<3.1
 django-dotenv==1.4.1
-wagtail>=2.7,<2.8
+wagtail>=2.8,<2.9
 wagtailfontawesome>=1.1.3,<1.2
 Pillow>=6.2.0,<6.3