Add bootstrap scripts for engine:

• generators (copy js/css to main app)
• generate test app

Right now, if a user cancels from sofort, the whole order is canceled. That means the user already received an order confirmation, and will then receive another email with an order cancellation. That's a bad process for the user. Better would be:

• User goes to sofort, enters his data, and order is confirmed after he is redirected back. (that means emails only go out if user actually sucessfully enters data on sofort)
• If user cancels he is just redirected back to the confirm page.

We tried to implement this by introducing a before_action in CheckoutController:

## checkout_controller_decorator.rb
Spree::CheckoutController.class_eval do
  before_action :redirect_or_complete_order, only: :update

  def confirm_stripe_order
    params[:state] = "complete"
    update
  end

  private
    def redirect_or_complete_order
      if params[:state] == "confirm"
        if update_order
          if @order.payments.any? { |pt| pt.source&.redirect? }
            redirect_to solidus_stripe_sources.redirect_path(@order.number)
          end
        end
      end
    end
end

## routes.rb
Spree::Core::Engine.routes.draw do
  get '/checkout/confirm_stripe_order', to: 'checkout#confirm_stripe_order', as: :checkout_confirm_stripe_order
end

This is already pretty hacky, and it didn't work because redirect is only possible after payment is authorized, which happens when user confirms the order (in normal solidus flow). That'd mean we'd have to change the process even more, which is why we stopped doing this.

Still, long-term it makes sense to adjust this, but would take a major rewrite.

🤷‍♀️

Add german translation for templates.
Add proper text for _sofort_description template.

When user is returned from Stripe, and returns to the order's view, we aren't show no flash messages.
Add proper flash messages depending on current state of payment:

• pending
• in progress
• completed / failed

For now only unit tests available, add complex tests (features, mock stripe notifications etc.)
TODO:

• to test webhooks

SOFORT only works in 5 countries, do not show it for users from other countries.
Available countries:

• Austria
• Belgium
• Germany
• Italy
• Netherlands
• Spain