deps: Update wagtail to 2.7
deps: Update wagtail to 2.7

This version is a long term release, so lets pin it until we have
good reasons to update again (like the django 2.2 update before).

liqd/a4-meinberlin,liqd/a4-meinberlin,liqd/a4-meinberlin,liqd/a4-meinberlin