Add full app-blocking spinning wheel mode

Calin wrote:

Yes, we still need that overlay since there are a lot of operations which take some time and we don't want to allow user to "touch" anything until running operation is done.
Possible scenarios:

• Burst page:
  a) start a simulation (wait until burst is saved into DB and started in the backend)
  b) select an existing burst (wait until all burst paramteres/visualizers are loaded)
  c) delete burst

Stimulus / Local connectivity / Setting Model Parameters

• if user changes surface, it takes some time to reload data, so progress would help here too.

Also, there are a lot of Ajax calls to server which does not provide feedback to user so we could use overlay there.