Style cleanups for tangent left border and height

Tangent posts were moving around when the hover buttons were displayed,
sets a min-height so the buttons always fit without affecting post height.
Additionally apply the tangent left-border to only the first level of replies
rather than the existing broken border between related tangents

Change-Id: I29c73f5c02039ac27d59c283f398e83882b92b50
Merge "Style cleanups for tangent left border and height"