GitHub Auth: Username(s) as allowed organization(s)?

I'm wondering if we can support use of GitHub username(s) as values under the [github.allowedOrganizations field](https://github.com/brigadecore/brigade/blob/v2/charts/brigade/values.yaml#L69) in Brigade's Helm chart -- and Brigade would limit authentication to users with those usernames only.

I'd actually done this successfully a few months back, but it appears to no longer work. (Now attempts lead to: `Could not authenticate the request: User is not a member of any allowed GitHub organizations`)

Admittedly, it was already kind of a 'hack'/unintended feature. If we do want to enable such functionality, I would understand adding in the form of a new field, say, `allowedUsers`.

Start with charts/brigade/values.yaml at github.allowedOrganizations, then trace the GitHub authentication path that produces the reported error. Clarify whether usernames belong in a new allowedUsers field or the existing field, and define done as documented, tested authentication for permitted usernames without breaking organization-based checks.

Written by the indexing model from the issue text.