Improve oauth2 idp-initiated login (backport #13476) (backport #13502)

[mergify]

Proposed Changes

This is an improvement to the /login endpoint (rabbit_mgmt_login) used for idp-initiated logons in the management ui.

The previous implementation produced a warning message in the browser the first time the form was submitted. The warning message warned the user that the form was about to be submitted again. This was because the response page to the POST request contained a javascript code that refreshed the page.

With this new change, the rabbit_mgmt_login module instead replies with a redirect 302 including a cookie with the token. This cookie is very short-lived, when the user is redirected to the Overview page of the management, the cookie is already gone.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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This is an automatic backport of pull request #13476 done by [Mergify](https://mergify.com).

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This is an automatic backport of pull request #13502 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5e5521a has failed:

On branch mergify/bp/v4.0.x/pr-13502
Your branch is up to date with 'origin/v4.0.x'.

You are currently cherry-picking commit 5e5521a3c.
  (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)

Changes to be committed:
	modified:   deps/rabbitmq_management/include/rabbit_mgmt.hrl
	modified:   deps/rabbitmq_management/src/rabbit_mgmt_login.erl

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_management/src/rabbit_mgmt_oauth_bootstrap.erl

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[michaelklishin]

@MarcialRosales have I resolved this conflict correctly for v4.0.x? Any reasons not to backport this change?

[MarcialRosales]

You did resolve correctly the conflict. I just manually triggered the only failed CI job and it has passed.