Only show comments-time-machine-links after visiting the link

[fregante]

Fixes #1953
Closes #2618
Related to #2131
Ancestor of #1863
Prophecy of #1448 🪁

Test

To test this feature, click the test links in #2618 (the feature depends on comment date)

Also here are two pre-edited links to see what they look like:

Still exist: https://github.com/sindresorhus/refined-github/blob/master/source/features/comments-time-machine-links.tsx?rgh-link-date=2019-12-11T10%3A12%3A11Z
File deleted: https://github.com/sindresorhus/refined-github/blob/master/source/features/default-to-rich-diff.tsx?rgh-link-date=2019-12-11T10%3A12%3A11Z

Screenshots

If the file still exists, we can also support #1953 by getting the branch name from the page

If the file is deleted, the feature will link to HEAD and hope that the file exists there.