Unify doc explorer navigation around schema reference concept

[leebyron]

As defined in https://github.com/graphql/codemirror-graphql/blob/master/src/utils/SchemaReference.js - SchemaReference gives us a serializable way to refer to different parts of a schema.

Ideally this general utility is moved into a place where it can be leveraged more broadly (graphql-js perhaps) along with serialization and deserialization utilities.

This would make a better general public API for navigating to any element, and would give codemirror-graphql a better interface for interoperating with any kind of documentation browser.

It would also unlock issues like #77 by allowing a serialization of the currently viewed doc

[thomasheyenbrock]

This is probably rather low priority right now as we're planning to move from Codemirror to Monaco. Let's leave it open for now and reassess this when we actually moved to Monaco.