Alias Pixai model tag

BUR #267 has been rejected.

create alias pixai_(model) -> pixaidiffusion

It's the actual name of their in-house model and makes it easier to distinguish it from the name of the site itself.

The bulk update request #267 (forum #1833) has been rejected by @SONYA.

After discussions about this tag and the fact that pixai has multiple exclusive models, I decided that this BUR is unnecessary.