Add ability to create & update snapshots via REST API

[westonruter]

To build upon #63.

• Allow snapshot posts to be created and updated, ensuring that content is in the proper format as an object mapping setting IDs to setting params.
• Prevent editing of slug, since the UUID should never change.
• Add support for PATCH updates.
• Consider using REST API endpoints in Customizer when saving/updating snapshots.
• (optional) Add endpoints that use UUID instead of post ID. We really don't need regular post IDs since snapshots have UUIDs. Really a random PUT request could be made to create a snapshot if it just supplies a proper UUID.

[westonruter]

Deprecated in favor of https://core.trac.wordpress.org/ticket/38900