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/Header.module.css

@@ -104,6 +104,24 @@
   flex-shrink: 0;
 }
 
+.versionMismatchWarning {
+  padding-left: 15px;
+}
+
+.versionMismatchWarning::before {
+  content: "";
+  display: inline-block;
+  position: relative;
+  top: 2px;
+  width: 16px;
+  height: 16px;
+  mask-image: url("./icons/AlertTriangleFilled.svg");
+  mask-repeat: no-repeat;
+  mask-size: contain;
+  background-color: var(--warning);
+  padding-right: 3px;
+}
+
 @media (min-width: 800px) {
   .headerLogo,
   .roomAvatar,