Alternative approach for `comments-time-machine-links` #2618

The timemachine link is useful but I never click it unless I really need that version, and most of the time I don't know this until I click the main link.

I suggest dropping the icon (closes #2131) and, instead, waiting for the user to click the link.

On the resulting page:

• if the linked file still exists, add the timemachine permalink near branch-buttons:

  <img width="564" alt="linked" src="https://user-images.githubusercontent.com/1402241/70611422-c4dfc600-1c37-11ea-94d0-54509ffd43b6.png">

• if the linked file is gone, add the timemachine link somewhere on the 404 page

This also makes it easier to enable https://github.com/sindresorhus/refined-github/issues/1953 because we don't have to query each link on the page before adding the link: we just query the current page.

For this to be possible, instead of adding an icon to these links, we should add a ?linked-time=${timestamp} parameter so this information is available on the linked page.

Test

• Still exist: https://github.com/sindresorhus/refined-github/blob/master/source/features/comments-time-machine-links.tsx#L6-L8
• File deleted: https://github.com/sindresorhus/refined-github/blob/master/source/features/default-to-rich-diff.tsx#L19