Show last update of item data

TODO:

- [ ] check if this is too performance intensive if you have a lot of buy / sell listings (I currently only have ~ 8 million in my db)
- [ ] display it in a nice way (maybe a column "Last updated / refreshed xx minutes ago")
- [ ] maybe do something similar for recipes

In a perfect world where servers never crash one would probably not need this but in reality there can be technical issues so it would be nice to know if the price data is old.