Resolve "Relative links in Vulnerability modal not clickable"

What does this MR do?

This MR brings a small change to the isSafeUrl utility so it considers root-relative URLs safe, meaning that all these patterns are valid:

• http://gitlab.com/some/path
• https://gitlab.com/some/path
• /some/path

Additionally, the valuedFields computed property will force field field to be an URL pointing to vulnerability.blob_path (if available).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style 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
  • Cross-browser testing revealed that isSafeUrl does not work as expected on IE11 (neither on master or this branch), the fix involves upgrading core-js so we can use the URL() polyfill, that will have to be done in follow up MRs

Closes #9283 (closed)