Add more configuration for testing various features

[laymonage]

• Add missing fixtures for German and Arabic locales
• Add more filters to snippet viewsets
• Register Country model with a ModelViewSet instead of a SnippetViewSet, so we have an example of a ModelViewSet
  • Enable inspect view for CountryModelViewSet
• Add UserApprovalTask, so we have an example of different task types
  • Based on https://docs.wagtail.org/en/stable/extending/custom_tasks.html
• Fix verbose name of user-facing models
  • So we can make sure we're applying capfirst() in the right places in Wagtail core, and prevent unnecessary capitalisation in other places. See https://github.com/wagtail/bakerydemo/pull/478#discussion_r1482662326.