Allow some price specifiers for free puzzles

Puzzles that are not sold in any way or not directly sold (promos) should be able to have this information specified. The price would essentially count as 0 for the purposes of sorting or math, but would be labelled with speficif strings. For instance "Free" may apply to puzzles that are freely available to download and 3D print with no option to buy. "Promo" may apply to puzzles given away with other purchases like a toy in a kid's meal or at an event with still no option to purchase it after the fact.