Add System emails menu & list to admin panel

We need a section to list all System Emails to view the templates and
preview an example of what would be sent.

app/controllers/admin/system_emails_controller.rb

@@ -0,0 +1,7 @@
+class Admin::SystemEmailsController < Admin::BaseController
+
+  def index
+    @system_emails = %w(proposal_notification_digest)
+  end
+
+end