Use Mongo _id instead of UUID for primary key

[vnugent]

We use UUID as the unique ID for climb, area, user, media object, etc, both in external resources (url slugs) and internal primary keys. It's difficult to work with UUID in the backend since we need convert UUID-dash-format <--> Internal binary.

Todo:

Investigate migration paths (converting _id to ObjectID, incrementally update GQL layers and data sources