Context links for comments when applicable

[BoringBuffalo]

Add a 'context' button to the bottom of comments (if they aren't top-level comments) which links to the parent comment.

This is especially important in views of threads that are portions of comment trees, and also in the user/comments view, and the messages/notifications view.

[Polsaker]

The permalink link kind of already does this

[BoringBuffalo]

Oh, that's not very intuitive, since you have to click on the parent's permalink to get the context, and it's not what one expects it to do. It does save a button.