chore(ui): display branch in footer if present

app/views/layouts/application.html.haml

@@ -55,6 +55,8 @@
         %footer.siteContent__footer
           %ul.footer__links
             %li.footer__name
-              Powered by #{link_to "Postal", "https://postalserver.io", target: '_blank'} #{Postal.version}.
+              Powered by
+              #{link_to "Postal", "https://postalserver.io", target: '_blank'}
+              #{postal_version_string}
             %li= link_to "Documentation", "https://docs.postalserver.io", target: '_blank'
             %li= link_to "Ask for help", "https://discussions.postalserver.io", target: '_blank'