Raise action bar above read marker

Use `z-index` to ensure the action bar appears above over things, like the read
marker.

Fixes https://github.com/vector-im/riot-web/issues/9619

res/css/views/messages/_MessageActionBar.scss

@@ -26,6 +26,7 @@ limitations under the License.
     top: -18px;
     right: 8px;
     user-select: none;
+    z-index: 1;
 
     > * {
         display: inline-block;