Initial UI for sequence progress.

* change bottom border of links:
  red for not started
  yellow for in_progress
  green for done

* This should probably be designed at some point.
* Obvious problems: the yellow is not very visible, and lots of people are
  red/green color-blind.

lms/static/sass/courseware/_sequence-nav.scss

@@ -66,6 +66,26 @@ nav.sequence-nav {
         @include transition(all, .4s, $ease-in-out-quad);
         width: 100%;
 
+        &.progress {
+          border-bottom-style: solid;
+          border-bottom-width: 4px;
+        }
+
+        &.progress-none {
+          @extend .progress;
+          border-bottom-color: red;
+        }
+
+        &.progress-some {
+          @extend .progress;
+          border-bottom-color: yellow;
+        }
+
+        &.progress-done {
+          @extend .progress;
+          border-bottom-color: green;
+        }
+
         //video
         &.seq_video_inactive {
           @extend .inactive;