Linkify our features in comments and titles #3980

This is another meta-feature just for our repo.

This is a little tricky because it's in 2 parts:

• in comments, where features are already in code, this is as simple as:

    for code of select.all('code'); linkify code if it's a feature

  Example, right here: latest-tag-button
• in conversation titles they first need to be be parsed from `feature` has a problem and then linkified. This would probably have to be part of format-discussion-titles Example URL: https://github.com/sindresorhus/refined-github/issues/3979

Note: this linkification should not happen everywhere parse-backticks operates, because in most cases those are already links.

Before / after

<img width="305" alt="Screen Shot 1" src="https://user-images.githubusercontent.com/1402241/107891632-8c93e180-6ee5-11eb-82eb-8a2ff6f7ebf5.png">

<img width="300" alt="Screen Shot 2" src="https://user-images.githubusercontent.com/1402241/107891628-8aca1e00-6ee5-11eb-9baa-31b7f793802c.png">