project: switch license from GPLv3 to AGPLv3

Closes #39 (closed).

Require explicit formal approval from all past contributors:

• Marin Hannache (@mareo)
• Sevan Murriguian-Watrin (@sevan)
• Arthur Outhenin-Chalandre (@arthur)

Approval must be committed in the repository as an empty GPG-signed commit with the following message :

project: switch license from GPLv3 to AGPLv3

I approve the relicensing of my contribution to this project from GPLv3
to AGPLv3.

Signed-off-by: firstname lastname <email>