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.
This adds a new framework for accessible dialogs that will eventually replace bootbox. Under the hood, it uses the a11y-dialog package and an in-repo Ember addon. See PR for usage details.
