Improve oauth2 idp-initiated login (backport #13476)

[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).