Ping the existing reference site on plugin activation

[balessan]

The idea is, on plugin activation, to send a GET request to the /sites/ url of the reference site (virtual-assembly i guess ? ) and to retrieve the currently know list of sites, to save them to.

Based on this list, we could also then send a POST request to the /sites/ url of each of them so they add the new site to their own list.

Specialisation of the site API so I can close the other issue

[balessan]

Tu as pu avancer @SebLemoine ? ;-)