Increase vertical offset of post-edit confirmation in MonoBook

At an offset of 3em from the top, the confirmation message overlies MonoBook's
tabs. If the user already knows that they want to use one of the tabs to
navigate away from the article page, she must wait for the confirmation to
fade out so that the obscured tabs are again visible and clickable. Increasing
the offset to 6em seems to fix it.

Bug: 41240
Change-Id: Ifc09523858adf47b7f321d9e29dcb53d290e9d81
Merge "Increase vertical offset of post-edit confirmation in MonoBook"