MM-8607 Add ability to turn off non-critical services when under load (#13212)

* MM-8607: add ability to turn off non-critical services under load

* server busy invalid param unit tests

* MM-8607: rename server busy endpoints

* MM-8607: handle case where App not initialized

* MM-8607: additional unit test cases per feedback.

* MM-8607: use decorator to check isbusy when adding endpoint route

* MM-8607: rename endpoints, use struct for json

* Update api4/system.go

Fix misspelled log output

Co-Authored-By: Saturnino Abril <saturnino.abril@gmail.com>

* MM-8607: fix i18n order; max seconds for server busy expiry

wsapi/websocket_handler.go

@@ -59,3 +59,7 @@ func (wh webSocketHandler) ServeWebSocket(conn *app.WebConn, r *model.WebSocketR
 func NewInvalidWebSocketParamError(action string, name string) *model.AppError {
 	return model.NewAppError("websocket: "+action, "api.websocket_handler.invalid_param.app_error", map[string]interface{}{"Name": name}, "", http.StatusBadRequest)
 }
+
+func NewServerBusyWebSocketError(action string) *model.AppError {
+	return model.NewAppError("websocket: "+action, "api.websocket_handler.server_busy.app_error", nil, "", http.StatusServiceUnavailable)
+}