* Rename `app` helper to `resolve`, deprecate old version
* Rename $this->app to $this->container in service providers
We no longer couple Flarum\Foundation\Application to the Laravel container; instead, we use the container separately. Changing our naming to reflect that will make things clearer.
This lets us register the former during installation, where the
latter is not yet registered.
That, in turn, means we can finally re-enable the StartSession
middleware in the installer app, which we need to log in the new
admin user when installation is complete.
