Save/restore identities in json for "course administrators"

[mquinson]

It should be possible to dump and download the identities as a json file, and also to import a bunch of new identities from a json file.

That's a very sensitive information, and at first I was reluctant to make it easy to download those informations, but at the same time, you want to have a test server where you check that your changes work before commiting, and you need to have real info on that server. Also, that functionality would be interesting when the main server is not available (its current instance is firewalled from the wireless network of our school).

I guess that in a perfect world, we would have to create a new role, like "course administrator", who is allowed to dump those info while regular teacher are not. If so, the "My students" page should probably not display the UUID of the students. There is no need for that info there anyway.

[Cedric-Huguenin]

Restoration can be temporarily performed with the conf/test-data.yml file.