Drop django-redis, in favour of Django's own Redis backend,
because wagtail-cache no longer suppors django-redis.
Drop hiredis, because we got this error when using it with Django's
own Redis backend:
ImportError: Module "redis.connection" does not define a "HiredisParser" attribute/class
Add unique cache URLs for tests, to avoid conflicts with the site's cache
Fix a few errors caused by deprecated things being dropped in Wagtail 5
Fix icons for Wagtail 5 (some of which were already broken in v4)
ImportError: Module "redis.connection" does not define a "HiredisParser" attribute/class