Improve navigation by adding the ability to put the currently selected comment on top of the page

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Is your feature request related to a problem? Please describe.
When you move down a page, you reach the point where you don't see the upcoming items on the screen. It would be nice if we could have a new navigation feature to put the currently selected item on top of the visible window.

Describe the solution you'd like
A new key press event, that puts the currently selected item on top of the visible window.

Describe alternatives you've considered
Moving a whole page down (by pressing n) is somewhat similar to this request, but it currently has other problems, described in #682.