Create organization behind feature flag

In Cells 1.0 we will limit who and how an organization can be created. These are the two cases when an Organization can be created:

1. A user has the (new) feature flag allow_organization_creation enabled.
2. A new user registers and elects to create an organization. User Registration: free or trial signup with co... (&12604) has more details.

This issue is to enable the case of 1 above. The allow_organization_creation feature flag does not exist and needs to be created.

Original discussion from #437541 (comment 1763605912)

Implementation guide

Backend

Add the feature flag to app/services/organizations/create_service.rb.

Frontend

Add the following to app/controllers/organizations/organizations_controller.rb

before_action only: [:index] do
  push_frontend_feature_flag(:allow_organization_creation, current_user)
end

Add conditional to that checks gon.features.allowOrganizationCreation in app/assets/javascripts/organizations/index/components/app.vue#L81 to hide New organization button