Add new enrollment message to the dashboard

This is related to the Destination edX Story: https://openedx.atlassian.net/browse/ECOM-403

Here's a screen shot of this, as-is:
![image](https://cloud.githubusercontent.com/assets/235752/4444488/42df33ea-47f1-11e4-81fb-58b75622d0fa.png)

@wedaly @dianakhuang @rlucioni @marcotuts  FYI this is not for review, just informational. 

Note: Style changes for messaging are going to be kept in a separate story, since we want to change the styles for all messaging on the dashboard. This PR will have the current message styles and be disabled by default (with a default configuration time delta of 0 seconds, the message will never be displayed)
The behavior here is a little subtle if `enrollment.created = None` (which is possible because the field is nullable).  These enrollments will be filtered out of the list.  This is the correct behavior, but it might be good to add a comment.