Set default audio license to CC BY
Set default audio license to CC BY

Solves #2848.

When a new user registers to Tatoeba, the audio license is set by
default to "No license for offsite use". Our Terms of Use state
however that the default is CC BY, meaning that we should have
CC BY 4.0 as the default value.

> La mention BY est également la condition de base pour les phrases
> audio, qui peuvent être contribuées sous différentes licences
> Creative Commons, impliquant d’autres conditions.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2