Open external links in new tab (fixes #4314)

[rohit-bharmal]

• Add Hugo link render hook so markdown external links get target=_blank and rel=noopener noreferrer
• Add rel=noopener noreferrer to navbar and feedback partials for external links

Closes #4314

[google-oss-prow]

Hi @rohit-bharmal. Thanks for your PR.

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[google-oss-prow]

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[Arhell]

/ok-to-test

[rohit-bharmal]

@Arhell Did you get some time to review this PR ? If not, could you please review this PR ?