Add commands for Admins to add GitHub members

[jb3]

• Add new commands for Admins to add GitHub members to the Staff team (and organisation)
• Remove unused Grafana GitHub team sync (replaced by LDAP)
• Add exemption to allow Admins to use GitHub management commands.

[jb3]

merging this completely untested

[sentry]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ ExtensionFailed: Extension 'arthur.exts.github.man... in production

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