New feature: Item data sets

Item datasets are now available on the items page. These are sets of regular foods and drinks that can be easily imported to your item database.

Currently there is only a "Fruits, vegetables and nuts" set available, but I'm adding some generic ones in the near future. Later on I plan to let users make and share datasets as well.

This set is now available in the registration form as well so new users can start with some default items.