Allow non- @gsa.gov users to GSA GitHub teams

issue discussed here:
https://github.com/18F/tts-tech-portfolio/issues/69

As a TTS system owner working across GSA/18f github orgs there is inconsistency in how we manage external (or non @gsa.gov users).

18f supports adding them to teams, which makes onboarding/offboarding easier to manage (https://handbook.18f.gov/github/#working-with-outside-collaborators) vs GSA org we need to grant access at the repository level, which is not a best practice for user management (as turnover is harder to manage this way). cc @afeld @gbinal @adborden