GitHub Organization membership based OAuth authentication support

[adamlabadorf]

Hi,

First thanks so much for your hard work making this excellent software!

I have a use case where I want to control user access using GitHub organization membership. The desired functionality was to add a list of allowed GitHub organization names at startup via environment variable config to the oauth flow and only allow access to those in certain organizations, rejecting otherwise.

I already have a base implementation of this feature that works but needs more polish. It makes these changes:

• reads comma delimited github organization names from the GITHUB_ALLOWED_ORGS environment variable, with special 'all-orgs' value to disable the behavior (blank environment variable rejects everybody)
• adds the read:org scope to the github oauth.js endpoints if GITHUB_ALLOWED_ORGS is anything other than 'all-orgs'
• adds a custom callback function in api/strategies/githubStrategy.js that queries user's organizations and authenticates if there is at least one org in common with the configured org names

I didn't see this feature on the roadmap. If there is interest in accepting this feature I'll make a pull request after I've finished polishing and implementing tests and documentation.

Happy for any ideas, guidance, or suggestions.