Fix header styling

Fix the double bottom border in the header (the additional not 100%
long line) on special pages.

Get rid of #mw-mf-header and use .header everywhere. Get rid of
.escapeOverlay and use .cancel instead.

Change-Id: I8fe0f3df1f0443fcbd94e9b2dfaa17af319e245c
Merge "Fix header styling"