feat(webapp): add SSE hint to footer

webapp/src/App.vue

@@ -57,6 +57,7 @@
           <b>deSEC e.V.</b>
         </div>
         <div class="d-sm-flex flex-row align-right py-2">
+          <div class="px-2 grey--text text--darken-1">powered by <a class="grey--text" href="//securesystems.de/" style="text-decoration: none">SSE</a></div>
           <div class="px-2"><a href="//github.com/desec-io/desec-stack/">Source Code</a></div>
           <div class="px-2"><router-link :to="{name: 'impressum'}">Impressum (Legal Notice)</router-link></div>
           <div class="px-2"><router-link :to="{name: 'privacy-policy'}">Datenschutzerklärung (Privacy Policy)</router-link></div>