Allows anonymous users to download the calendar file. Before, they were given the option, but it would fail silently with a `TypeError: Cannot read properties of null (reading 'user_option')`.
As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.
This commit also introduces an eslint rule to prevent importing from
the old I18n path.
For themes/plugins, the old 'i18n' name remains functional.
User options were serialized at the root level of CurrentUserSerializer,
but UserSerializer has a user_option field. This inconsistency caused
issues in the past because user_option fields had to be duplicated on
the frontend.
