Make it easier to update credentials in import_url

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GitHub is changing the format of their OAuth2 and personal access tokens: https://github.blog/2021-04-05-behind-githubs-new-authentication-token-formats/

Right now import_url may store legacy temporary OAuth2 tokens or the older personal access tokens. We don't have a way to refresh or update these tokens easily at the moment, even with the API (#2549 (closed)). Currently the only way to update this is to delete the mirror and recreate it.

This would be useful in general for other pull/push mirrors, especially since it's easy to get the URL wrong the first time.