The actual navbar height is 3.5rem.

/fyi @mdo

Note: The selector in dashboard.css that used .page-header was superfluous
    because the margin that it zeroed was already zero even without that declaration.

[skip sauce]

If you add an "active" class to one of the <li> elements in the top navigation, and style it so that active tabs have a different color, it becomes clear that there is a 1 pixel stray line under the navigation. This style fixes that.

Follow up to 1bdd59b18c2d1b2409d0422a60e8c007053202df, to better position the sidebar so the potential scrollbar doesn't get hidden by the navbar

Fixes #12435