Authenticate on confirmation

What does this MR do?

When a user clicks on the link in the confirmation email, we now authenticate them immediately. 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.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56517

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

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

PS: Sorry for the strange commit history in the audit log. There was an oversight and I pushed from the wrong folder.