Consider adding separators to thread nav buttons (Follow-up from "Fix radii of MR thread nav buttons")

The following discussion from !205942 (merged) should be addressed: !205942 (comment 2770741129)

Related, this whole control is very similar to a label button, which has separation between the buttons to clarify the label and the buttons. Trying to adapt that to here, that would be:

Label button example	All borders	Border between label and button

It's strange that we are not applying this component here, as we do in other places, like the commits views.