Save Code Now does not work for GitHub origins while GitHub's API is unavailable

GitHub's API can be unavailable for various reasons (upstream is unavailable, temporary network failure, blocked by a firewall, blocked by local policies such as being a third-party XHR, ...)

When this happens, the Save Code Now form should either report an error, or submit the request to the server anyway.

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Migrated from T4055 (view on Phabricator)