Work around incompatibilty between govuk-frontend-jinja and Flask 2

This line:
ddbe208a97/govuk_frontend_jinja/flask_ext.py (L22)

Raises `KeyError: 'extensions'` when using Flask 2

I think this is because there are no default Jinja extensions in Flask
as of pallets/flask@81ba6c2 in accordance with
https://github.com/pallets/jinja/issues/1203

So we need to manually add an `extensions` field to the `jinja_options`
`dict` if one doesn’t exist already.

Issue raised here: https://www.github.com/Crown-Commercial-Service/govuk-frontend-jinja/issues/66

app/__init__.py

@@ -162,6 +162,9 @@ def create_app(application):
 
     init_app(application)
 
+    if 'extensions' not in application.jinja_options:
+        application.jinja_options['extensions'] = []
+
     init_govuk_frontend(application)
     init_jinja(application)