Clean up the Django ORM usage a bit

lms/templates/discussion/_user_profile.html

@@ -3,7 +3,8 @@
 <div class="user-profile">
   <div class="sidebar-username"><a class="learner-profile-link" href="${learner_profile_page_url}">${django_user.username | h}</a></div>
   <div class="sidebar-user-roles">
-      ${_(', ').join(sorted(set(map(_, [role.name for role in django_user.roles.all()]))))}
+    <% role_names = django_user.roles.order_by("name").values_list("name", flat=True).distinct() %>
+    ${", ".join(_(role_name) for role_name in role_names)}
   </div>
   <div class="sidebar-threads-count">${ungettext('%s discussion started', '%s discussions started', profiled_user['threads_count']) % span(profiled_user['threads_count']) | h}</div>
   <div class="sidebar-comments-count">${ungettext('%s comment', '%s comments', profiled_user['comments_count']) % span(profiled_user['comments_count']) | h}</div>