Replace raster image gradients (before base64 embedded) for modern
browsers by CSS gradients. Only relying on conventional image URLs
in browsers that don't support linear-gradient.
Also updating color to Wikimedia Design Style Guide color and
removing obsolete *-ltr/*-rtl images that have been introduced in
MW core 1.16.0beta1 but have been replaced by unified 'portal-break'
image afterwards.
Bug: T63099
Bug: T121730
Change-Id: Id0ab5eb7d4a5751316ce750aff2dffbda62f6b09
