Sign in user on email confirmation

What does this MR do?

• Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/56517
• Previous CE MR: gitlab-foss!31699 (closed)

When a user clicks on the link in the confirmation email, we now authenticate them immediately within the first 24 hours. This improves the on-boarding flow for our users, as it removes another obstacle to start using GitLab.

This MR introduces some risk since it authenticates a user. I've added tests in the controller to test if we we should authenticate the user (based on the discussion in https://gitlab.com/gitlab-org/gitlab-ce/issues/56517) as well as added audit logging.

We only authenticate the user if:

1. They have not confirmed their mail yet (have not clicked on the link)
2. They clicked the link within 24 hours. The timeframe is up for discussion.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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