Implement a proof of concept demonstrating permissions model

Created by: pjlast

Implement a small proof of concept that clearly demonstrates the permission model's capability of solving the "embed a code insight into a notebook" permissions scenario. A doc describing the plan for the PoC can be found here

• Create repository for PoC
• Set up DB with migrations
• Implement read and write API for permissions
• Implement demo permission flow for an example use case