Write RFC for long term plan on "Control users that sign in to SG"

Created by: kopancek

We need a long term plan and technical roadmap for the "Control users that sign in to sourcegraph" feature.

Expected outcomes

• RFC with technical roadmap written and reviewed by our team, at least a few engineers outside of the team, security, etc.
• RFC will contain: definition of the problem, architecture of the long term solution, technical proposal and steps on how to achieve the proposal