Avoid Devise "401 Unauthorized" responses

What does this MR do?

Avoid Devise "401 Unauthorized" responses

Instead, in GitLab, we want to redirect to the sign-in page. This happens for most requests, but when Devise can't work out the request format, or for certain other kinds of request, it will send a very poor 401 Unauthorized instead.

This is acceptable in certain cases (XHR or git clone requests, say), but should not happen for user-visible requests - they should be redirected to sign in, instead.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #62614 (moved)