Fix Jira app can only load public namespaces

What does this MR do and why?

This fixes a bug that allowed the GitLab for Jira app only to load public namespaces.

The request that fetches the namespaces didn't contain the authorization header. This MR is based on @justin_ho's proposed solution.

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Numbered steps to set up and validate the change are strongly suggested.

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Related to #388152 (closed)