Replace portal break image by SVG gradient

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MW core 1.16.0beta1 but have been replaced by unified 'portal-break'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
(cherry picked from commit 5ed9bf8be6)

components/navigation.less

@@ -92,8 +92,10 @@
 		.body {
 			margin-left: @margin-left-nav-main-body;
 			padding-top: 0;
-			background-image: url( images/portal-break.png );
+			background-image: url( images/portal-separator.png ); // Support: IE 8 & 9, Fx 3.6-15, Safari 5.1-6, Chrome 10-25
+			background-image: linear-gradient( to right, transparent 0, #c8ccd1 35%, #c8ccd1 70%, transparent 100% ); // Standard (Firefox 16+, IE 10+, Safari 6.1+, Chrome 26+)
 			background-repeat: no-repeat;
+			background-size: 100% @border-width-base;
 
 			ul {
 				list-style: none none;