hgweb: make followlines button absolutely positioned It used to have position: absolute only on annotate page, but it makes sense to have it everywhere, because the button shouldn't affect other elements at all. Especially since the button has a set height, which meant that for certain smaller fonts source lines were changing their height on hover. Note that the button doesn't set any of the usual properties that accompany absolute position (top, right, bottom or left). These properties would position the button without any account for source line padding. Instead, margins are used (the button already has all margins defined, they do the job).