Initialize observables when kernel is ready
Initialize observables when kernel is ready

- notice app_initialized.NotebookApp is triggered way before kernel load:
  * and even before loading of notebook, which in turn on success calls load_notebook_success, which only initiates kernel start (kernel.start_session()).
- and observable.start(), as I understand, relays on a kernel to be initialized.

a quickfix is to simply change this initialization to happen on kernel_ready.Kernel. This should, hopefully, fix the mysterious cases when progressbar is non-visible!

andypetrella/spark-notebook,maasg/spark-notebook,radek1st/spark-notebook,antonkulaga/spark-notebook,radek1st/spark-notebook,radek1st/spark-notebook,andypetrella/spark-notebook,deanwampler/spark-notebook,spark-notebook/spark-notebook,meh-ninja/spark-notebook,minyk/spark-notebook,0asa/spark-notebook,maasg/spark-notebook,minyk/spark-notebook,deanwampler/spark-notebook,antonkulaga/spark-notebook,0asa/spark-notebook,spark-notebook/spark-notebook,meh-ninja/spark-notebook,maasg/spark-notebook,0asa/spark-notebook,andypetrella/spark-notebook,minyk/spark-notebook,radek1st/spark-notebook,0asa/spark-notebook,meh-ninja/spark-notebook,spark-notebook/spark-notebook,antonkulaga/spark-notebook,minyk/spark-notebook,andypetrella/spark-notebook,maasg/spark-notebook,spark-notebook/spark-notebook,deanwampler/spark-notebook,meh-ninja/spark-notebook,deanwampler/spark-notebook,antonkulaga/spark-notebook