Make "Learn more about" links for security scanning popovers open in new tab

What does this MR do?

When viewing a merge request, the security scanning panel has question bubble icons that show a popover with a "learn more about" link. These links now open in a new tab and the link itself shows an external link icon. A unit test was also added for the mixin that generates the external links.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] NOT REQUIRED Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] NO STYLE CHANGES Style guides
• [-] NO DB CHANGES Database guides
• Separation of EE specific content

Availability and Testing

Risk is minimal. Cross-browser testing required.

• 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 #13333 (closed)