Reuse sign-in OAuth workflow for code host connections
Created by: artemruts
Description
To reuse existing "sign-in with" OAuth workflow for code host connections, we need to have a secure way to tell the sign-in-workflow to bind given code host (user external account) to user and not with an email but with a special ID X, where X could be an opaque user ID.
Otherwise we need to require users to add their email address (of what they have in the code host) before creating code host OAuth connection. If user won't have an email address, we may end up creating a new Sourcegraph account.