Add unique card background for Deck of Trials cards.

[acourrau]

Deck of Trials cards thematically are supposed to be powerful cards that are limited in the number that get added to your deck. I think with the legendary cards getting a special card background, adding something special and unique for Deck of Trials cards would be very good to have.

[acourrau]

Neablis mentioned that we had a similar concept for the Wanderer, and we could do something similar for the Deck of Trials cards. See: https://discordapp.com/channels/329758756076519436/638766476597264394/775580977056579604