Use seperate table for item meta. Add Fields to Categor, Store and Item. 

## ItemMeta
- Instead of JSONField, use a meta table. 

## Item
- Add `featured` field
- Add `description` field

## Category
- Add `slug` for shortname
- Add `icon` to specify icon file name. 

## Store
- `slug` for shortname, `name` for display name.