add sidebar-visible mixin

Author: lazabogdan

src/sass/mixins/_sidebar-layout.scss

@@ -79,4 +79,12 @@
 			}
 		}
 	}
+}
+@mixin sidebar-visible($breakpoint: '') {
+	@if $breakpoint != '' {
+		$breakpoint: -#{$breakpoint};
+	}
+	&.sidebar-visible#{$breakpoint} {
+		display: block;
+	}
 }