add a loader when simulation jar is uploaded

[miccar]

in the System Management UI: add a loader while a simulation jar is uploading (when Upload button is clicked)

[bissim]

My attempt to properly show the loader with the effective status of the simulation jar upload up to now failed.

Discussing the issue with @spagnuolocarmine we agreed on a possible solution: extract the file field from the simulation form and put it into a separate one, in order to retrieve the progress status of submission needed by the paper-progress element.