Notify Sentry when repo activation fails

https://trello.com/c/VcZ1JoDq/225-notify-sentry-when-repos-fail-to-activate
do not retry Octokit::Unauthorized
Add missing expectation
Use cached GitHub responses when 304 is returned

https://trello.com/c/1qeitvaD/204-address-rate-limiting
Add default style guide test coverage