GitHub Teams audit bot

This bot would audit the project's GitHub Team memberships against the team members reported on Astropy Team webpage. If any inconsistency is found, bot would open issue(s) on the relevant repository.

Possible cadence: Scheduled, when GitHub Team change is made, when repo access is updated.

Proposed by: @astrofrog

**Blocked by:** astropy/astropy.github.com#393

Also see: https://github.com/astropy/astropy.github.com/issues/323#issuecomment-689802741

Start with the blocked astropy/astropy.github.com#393 issue and the discussion in astropy.github.com#323. Clarify how the Astropy Team webpage reports membership, which GitHub Teams and repositories are in scope, and which cadence should trigger audits. Done means the bot detects inconsistencies and opens issues in the relevant repository or repositories.

Written by the indexing model from the issue text.