Update for SG 3.1 - Scopes and Collections

There are a few customers who use this tool to work with SG instead piecing together HTTP calls via SWAGGER page in SG's docs.
Could some one update to show off new features of scopes and collection setting. Also a few examples of how to update/create SG users using Scopes and collections.

Start with the existing Postman collection and compare its Sync Gateway requests with the 3.1 scopes and collections features described in the issue. Update the collection with examples for creating and updating users using scopes and collections; done means the new requests and examples are present and usable.

Written by the indexing model from the issue text.