Moved the item import to the v2 GW-API

This commit finishes the move to the v2/items-API of GW. We can now:
- nix the ItemDB-Queue (which happened, the workers will output a message and exit with code 1)
- use tools/update-items-from-api.php for item updates. The script runs about 5 minutes and creates new (Sub)Types if needed.

I also adapted the Readme.