Add TCGPlayer Sale Notifications

https://docs.tcgplayer.com/docs/external-kickbacks-api

These should be unobtrusive, only shown on the card price dialog above the existing TCGPlayer link.

The user should also be able to opt-in to notifications shown in an in-app dialog, shown when a sale is active and a notification for that sale hasn't been shown yet. This opt-in will be a shared preference, and also displayed once upon app update.