Add Admin System Emails view & first system email

Admins need to be able to see what a particular System Email looks like
with dummy data.

Also adding the first system email to be managed: the proposal
notification digest.

In case the current admin doesn't have any Notifications from
 ProposalNotifications there will be a crash. We'll solve this in later
 PR's with a system to "inject" sample text in email templates without
 having to generate records in the database.

app/controllers/admin/system_emails_controller.rb

@@ -1,7 +1,22 @@
 class Admin::SystemEmailsController < Admin::BaseController
 
+  before_action :load_system_email, only: [:view]
+
   def index
     @system_emails = %w(proposal_notification_digest)
   end
 
+  def view
+    case @system_email
+    when "proposal_notification_digest"
+      @notifications = Notification.where(notifiable_type: "ProposalNotification").limit(2)
+      @subject = t('mailers.proposal_notification_digest.title', org_name: Setting['org_name'])
+    end
+  end
+
+  private
+
+  def load_system_email
+    @system_email = params[:system_email_id]
+  end
 end