By @thisisnsh: Restore the pre-login page after an idp_initiated OAuth login (backport #16961)

[mergify]

This is #16957 by @thisisnsh with a small usability fix from me.

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This is an automatic backport of pull request #16961 done by Mergify.

[mergify]

Cherry-pick of 8d7333e has failed:

On branch mergify/bp/v4.3.x/pr-16961
Your branch is up to date with 'origin/v4.3.x'.

You are currently cherry-picking commit 8d7333e115.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_management/priv/www/js/main.js

no changes added to commit (use "git add" and/or "git commit -a")

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