Add support for org.matrix.custom.text.html

This format will remain undocumented as it is not yet suitable for
introduction into the specification.

webclient/components/matrix/event-handler-service.js

@@ -460,6 +460,7 @@ function(matrixService, $rootScope, $q, $timeout, mPresence) {
                         handleRoomAliases(event, isLiveEvent);
                         break;
                     case "m.room.message":
+                    case "org.matrix.custom.text.html":
                         handleMessage(event, isLiveEvent);
                         break;
                     case "m.room.member":