GNS3v3: webui/Add new template/Import an appliance file

[llevier]

"Click to import Appliance", selected "alpine-linux.gns3a"

gns3 answers: "Image 'upload/alpine-linux.gns3a' not found"

Despite just after, the file is detected present (with the .qcow2 of other appliances) when checking.

[grossmj]

This is currently broken. We need to figure out what we want to do on back-end side first before we do something in the web-ui