LMS: adds in additional abstract border-variables with logical scale

lms/static/sass/base/_variables.scss

@@ -240,6 +240,10 @@ $border-color-1: rgb(190,190,190);
 $border-color-2: rgb(200,200,200);
 $border-color-3: rgb(100,100,100);
 $border-color-4: rgb(252,252,252);
+$border-color-l1: $m-gray-l1;
+$border-color-l2: $m-gray-l2;
+$border-color-l3: $m-gray-l3;
+$border-color-l4: $m-gray-l4;
 
 // MISC: links and buttons
 $link-color: $blue;