Require users to provide the initial transition

[bertdeblock]

The main motivation for this change is the removal of the ember-permissions instance initializer.
The initializer eagerly looks up the permissions service, which in turn eagerly looks up the router service, making it
impossible to stub any of the two in tests.

Before:

import Route from '@ember/routing/route';
import { service } from '@ember/service';

export default class ProtectedRoute extends Route {
  @service('permissions') permissionsService;

  beforeModel() {
    this.permissionsService.enableRouteValidation();
  }
}

After:

import Route from '@ember/routing/route';
import { service } from '@ember/service';

export default class ProtectedRoute extends Route {
  @service('permissions') permissionsService;

  beforeModel(transition) {
    this.permissionsService.enableRouteValidation(transition);
  }
}