fix: preview mode navigation previous button (that uses legacy UI)

Legacy UI: Previous button should use prev_url if it in first
position. In studio UI, it should always use prev_url. To make this
possible we checking length of contents which is always zero for studio
and more than 0 for legacy UI.

xmodule/js/src/sequence/display.js

@@ -364,7 +364,7 @@
 
             if ((direction === 'next') && (this.position >= this.contents.length)) {
                 targetUrl = this.nextUrl;
-            } else if ((direction === 'previous')) {
+            } else if ((direction === 'previous') && (this.position === 1 || this.contents.length === 0)) {
                 targetUrl = this.prevUrl;
             }