As a developer I want to fetch data of an item, so I can use elements I need in my application

Notes

• See a 2020 suggestion for GET /entities/items path: https://wmde.github.io/wikibase-rest-api-proposal/#/items/get_entities_items
• When creating the stories, be sure to cover both getting a single and a list of items

Break down into:

• create GET /entities/items that returns all item data and ignores If-semantics, and authentication/rate limiting
• handle _fields parameter
• handle If-None-Match, and/or If-Modified-Since
• handle authentication
• 404 response for missing item