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/button/Button.jsx

@@ -41,6 +41,7 @@ export const variantToClassName = {
   iconCopy: [styles.iconCopyButton],
   secondaryHangup: [styles.secondaryHangup],
   dropdown: [styles.dropdownButton],
+  link: [styles.linkButton],
 };
 
 export const sizeToClassName = {

src/button/Button.module.css

@@ -207,3 +207,10 @@ limitations under the License.
 .lg {
   height: 40px;
 }
+
+.linkButton {
+  background-color: transparent;
+  border: none;
+  text-decoration: underline;
+  cursor: pointer;
+}