Display registered when already registered for a course.

lms/static/sass/_course_about.scss

@@ -97,6 +97,20 @@
               color: rgb(255,255,255);
             }
           }
+
+          span.register {
+            @include button(shiny, lighten(grayscale($blue), 50%));
+            @include box-sizing(border-box);
+            @include border-radius(3px);
+            color: #888;
+            display: block;
+            font: normal 1.2rem/1.6rem $sans-serif;
+            letter-spacing: 1px;
+            padding: 10px 0px;
+            text-transform: uppercase;
+            text-align: center;
+            width: flex-grid(12);
+          }
         }
       }
 

lms/templates/portal/course_about.html

 <%! from django.core.urlresolvers import reverse %>
+<%! from student.models import CourseEnrollment %>
 <%namespace name='static' file='../static_content.html'/>
 <%block name="js_extra">
   <script src="${static.url('js/course_info.js')}"></script>
@@ -16,7 +17,11 @@
 
         <div class="main-cta">
           %if user.is_authenticated():
-            <a href="${reverse('enroll', args=[course.id])}" class="register">Register</a>
+            %if not CourseEnrollment.objects.filter(user=user, course_id=course.id).exists():
+                <a href="${reverse('enroll', args=[course.id])}" class="register">Register</a>
+            %else:
+                <span class="register disabled">Registered</span>
+            %endif
           %else:
             <a href="#signup-modal" class="register" rel="leanModal" data-notice="You must register in order to enroll">Register</a>
           %endif