Add warning if incompatible versionsd are being used

This will probably be overly sensitive until we start timing out
member events (ie. https://github.com/matrix-org/matrix-js-sdk/pull/2446
lands) because lots of calls might have old member events from people
who've joined previously.

src/index.css

@@ -29,6 +29,7 @@ limitations under the License.
   --accent-20: #0dbd8b33;
   --alert: #ff5b55;
   --alert-20: #ff5b5533;
+  --warning: #e8bf37;
   --links: #0086e6;
   --primary-content: #ffffff;
   --secondary-content: #a9b2bc;