feat: allow OAuth success callback page to self-close when possible

Case Taintorrequested to merge
gitlab-community/gitlab-org/api/client-go:improve-success-callback-ux into main

This MR aims to improve the UX for OAuth. If the user has an active GitLab session then this MR makes it so that the opened callback page will close automatically.

  • If the user has an active session – the window will close automatically after 1.5s
  • If the user does not have an active session – they will land on the success page and be told that they need to close it manually

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