Add Preview Pending action to Admin System Emails

Some system emails need to be manually reviewed before being sent. This
new action allows admins to see a preview of all Pending to be sent
Proposal Notification Digest messages.

app/controllers/admin/system_emails_controller.rb

@@ -1,9 +1,11 @@
 class Admin::SystemEmailsController < Admin::BaseController
 
-  before_action :load_system_email, only: [:view]
+  before_action :load_system_email, only: [:view, :preview_pending]
 
   def index
-    @system_emails = %w(proposal_notification_digest)
+    @system_emails = {
+      proposal_notification_digest: %w(view preview_pending)
+    }
   end
 
   def view
@@ -14,9 +16,22 @@ class Admin::SystemEmailsController < Admin::BaseController
     end
   end
 
+  def preview_pending
+    case @system_email
+    when "proposal_notification_digest"
+      @previews = ProposalNotification.where(id: unsent_proposal_notifications_ids)
+                                      .page(params[:page])
+    end
+  end
+
   private
 
   def load_system_email
     @system_email = params[:system_email_id]
   end
+
+  def unsent_proposal_notifications_ids
+    Notification.where(notifiable_type: "ProposalNotification", emailed_at: nil)
+                .group(:notifiable_id).count.keys
+  end
 end