Add 'rel' attribute to default welcome page. (#5695)

add rel attribute as a precaution against reverse tabnabbing in future

changelog.d/5695.misc

+Add precautionary measures to prevent future abuse of `window.opener` in default welcome page.

synapse/static/index.html

@@ -48,13 +48,13 @@
     </div>
     <h1>It works! Synapse is running</h1>
     <p>Your Synapse server is listening on this port and is ready for messages.</p>
-    <p>To use this server you'll need <a href="https://matrix.org/docs/projects/try-matrix-now.html#clients" target="_blank">a Matrix client</a>.
+    <p>To use this server you'll need <a href="https://matrix.org/docs/projects/try-matrix-now.html#clients" target="_blank" rel="noopener noreferrer">a Matrix client</a>.
     </p>
     <p>Welcome to the Matrix universe :)</p>
     <hr>
     <p>
       <small>
-        <a href="https://matrix.org" target="_blank">
+        <a href="https://matrix.org" target="_blank" rel="noopener noreferrer">
           matrix.org
         </a>
       </small>