GitHub sync fails for users with expired invite

[augustuswm]

If a user in configs has been invite to the GitHub org in the past, but did not accept their invite and instead has let the invite expire, subsequent attempts to add the user to the organization will fail with 422 errors. This is seen in src/providers.rs:242. Currently this causes the sync function to return early with a failure.

This should likely be changed, as we are left currently with a partially provisioned user. Either the remaining provisioning steps could be run and GitHub access can be left in its current state or be manually triaged, or we can cancel the invite and re-issue upon checking that it is expired.