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.
Previously, we had a `showFooter` boolean on the application controller which would be set true/false in various routes by different routes/controllers. A global `routeWillChange` hook would set it `false` before every route transition, and the destination route/controller would have to set it `true` for the footer to show correctly.
This commit replaces that with a new 'declarative' system. Instead of having to set the value true/false manually, UIs which need the footer to be hidden can simply include the `{{hide-application-footer}}` helper in their template when needed. The helper/service will automatically keep track of all the current invocations of that helper, and only show the footer when there are 0 invocations.
This significantly simplifies things, and removes the need for many observers and controller injections, both of which are considered 'code smells' in modern Ember applications.
Godfrey Chan
David Taylor
Roman Rizzi
Robin Ward