Support PAT authentication for visual reviews

What does this MR do?

Due to security implications brought up on !27804 (comment 316217435), we decided to use Personal Access Token authentication for now to make Visual Reviews work on private/internal projects.

This also has the added benefit of giving public projects the option to require authentication thus tying up the feedback to an existing GitLab account.

Related discussion: #42750 (comment 319279512) Related MR on visual-review-tools: visual-review-tools!25 (merged)

This also updates the visual-review-tools to 1.6.1.

Solves #42750 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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