Change opacity of member list entries when their presence changes.

This is done by attaching a single room listener at the member list level and
then forceUpdate()ing the individual tiles as presence changes come in from
the JS SDK. This is more efficient than having hundreds of listeners attached
directly to the JS SDK (if we were to add a listener per tile).

skins/base/css/molecules/MemberTile.css

@@ -51,3 +51,11 @@ limitations under the License.
     overflow: hidden;
     text-overflow: ellipsis;
 }
+
+.mx_MemberTile_unavailable {
+    opacity: 0.75;
+}
+
+.mx_MemberTile_offline {
+    opacity: 0.5;
+}