pre-release versions displayed by default

This is just a heads-up for a bug noticed in the code [1]

Currently when accessing a url like:

https://apps.ubuntu.com/cat/applications/whatever/

it returns the app for the latest series, whether or not that's a pre-release series or not.

While discussing solutions for that, we said:
- check_all_latest will set the is_latest bit for the latest *released* distroseries
- find_best and find_best_or_404 will just check the is_latest bit or be removed entirely so that call-sites can just do: Application.objects.filter(package_name=package_name, is_latest=True) (or use get_object_or_404)
- Ensure check_all_latest is called at the end of import_all_app_install_data automatically.

[1] https://pastebin.canonical.com/68868/