vector: Move right tabs from behind to below left tabs

On small resolutions, instead of overlapping, the "right" one will be
displayed below the "left" one. It might overlap page's title or
sitenotice, but this is less bad than rendering critical functionality
unreachable.

This also cleans up the CSS slightly, positioning left and right nav using
the same constructs (margins instead of absolute positioning).

Vector extension's ext.vector.collapsibleTabs had to be adjusted to work
with new styles; see change Ie34dce9a9 in Vector.

Bug: 20234
Change-Id: Iedaebefc139ecd7ad48867626b67c52c6d00121d