Prevent multiple exports at the same time
On a UX point of view, it is very perturbating to be allowed to export an instance several times at the same time. As there is no information that the export is being done, a user could click several times on the "export" button, leading to a server overload We should :
- inhibit the button when an export is being done
- display a small spinner on the export button when an export is being done