Fix link for ad blocker.

Explore new courses links gets
disabled by ad blocker. Changing
class name to bypass this.

PROD-1348

lms/static/sass/multicourse/_dashboard.scss

@@ -531,7 +531,7 @@
           margin-bottom: $baseline;
         }
 
-        .ad-link {
+        .marketing-link {
           @include text-align(center);
 
           .btn-neutral {

themes/edx.org/lms/templates/dashboard.html

@@ -278,7 +278,7 @@ from student.models import CourseEnrollment
               <div class="advertise-message">
                   ${_("Browse recently launched courses and see what's new in your favorite subjects.")}
               </div>
-              <div class="ad-link">
+              <div class="marketing-link">
                   <a class="btn-neutral" href="${marketing_link('COURSES')}">
                       <span class="icon fa fa-search" aria-hidden="true"></span>
                       ${_("Explore New Courses")}