MDL-62180 course: Add distinctive class to activity navigation

Having a distinguishable class "activity-navigation" added to the activity
navigation allows that part to be easily styled. It could for example be
excluded from printing. Furthermore, this is providing consistency. Other
navigation elements have proper own classes as well.

course/templates/activity_navigation.mustache

@@ -64,7 +64,7 @@
         }
     }
 }}
-<div class="m-t-2 m-b-1">
+<div class="m-t-2 m-b-1 activity-navigation">
 {{< core/columns-1to1to1}}
     {{$column1}}
         <div class="pull-left">