Add polyfill to make PR table work in old browsers.

[robinlinden]

Big thanks to @cebe for noticing that the PR table is broken in e.g. Debian stable's Firefox. I tested this PR and verified that it works on IE and Edge.

This change is 

[fcore117]

Reviewed 1 of 1 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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[cebe]

not 100% sure as https://git-critique.herokuapp.com/hello/pulls is down but it fixes the original error.

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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[cebe]

Reviewed 1 of 1 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable