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.
1. actually call `popupAjaxError`, thanks :P
2. don't close a modal on error
3. use `extractError()` instead of manually joining error messages
4. …or passing just the error object to `this.flash`
