Add support for multiple GitHub User accounts

[testworksau]

Is your feature request related to a problem? Please describe.

We're migrating across to GHEC + EMU (from GHEC) and we cannot currently connect our GitHub Slack App to the new EMU enterprise (using /github signin) without disconnecting it from our existing GHEC.

https://docs.github.com/en/enterprise-cloud@latest/admin/identity-and-access-management/using-enterprise-managed-users-for-iam/about-enterprise-managed-users

An EMU user account only has read access to public repositories. Once we get to the point where we are completely migrated across to EMU, we'll have some users that require personal accounts for open source contributions either in our company open source org, or 3rd party open source repositories.

Those users will likely find it desirable to be able to subscribe to notifications for those public repositories.

Describe the solution you'd like The ability to connect more than 1 GitHub User Accounts

Describe alternatives you've considered Falling back to email :(

[rdohms]

Same here. We have partial migration to GHEC and I would like to subscribe to Orgs from GH and GHEC.

[raphaelflash]

We have the exact same issue. I would love to sign in with both my regular GitHub account, as well as my GitHub Enterprise account. At the moment it only works with either GitHub or GitHub Enterprise.