Update Items endpoints

[emilymdubois]

Refs https://github.com/osmlab/to-fix-backend/issues/143

• [x] Removes name property from schema (user-provided ID + metadata should suffice)
• [x] Updates GET /projects/:project/items endpoint
• [x] Adds POST /projects/:project/items endpoint
• [x] Updates GET /projects/:project/items/:item endpoint
• [x] Updates PUT /projects/:project/items/:item endpoint
• [x] Updates tests

cc @batpad