2015: clarify Free License for Wikimedia Commons

We should add a phrase in the Gallery clarifying that photos are available under CC BY-SA 4.0, Linux Day Torino 2015 contributors.

https://linuxdaytorino.org/2015/

This is useful to harden these photos, so Wikimedia Commons people do not delete these:

• https://commons.wikimedia.org/wiki/File:LDTO2015_-_pinguino_di_stoffa_di_Hachi.jpg
• ...

This issue depends on #28.