Linkify line numbers in "review blocks" in PR conversation #4572

1. Visit https://github.com/sindresorhus/refined-github/pull/4559#pullrequestreview-703841063
2. Click line 16
3. Nothing happens

<img width="305" alt="Screen Shot 11" src="https://user-images.githubusercontent.com/1402241/125929447-e7d3f950-1781-4f12-8af2-3a0dbaded37d.png">

Annoyingly enough it shows a pointer cursor but does nothing. New feature under "GitHub Shortcomings"

Notes:

• The number is injected via :before on a <td> element so wrapping that with a <a> might not be straightforward. Please look at how GitHub handles these numbers elsewhere.
• This must be accomplished via <a>, not a click handler