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)
Should we allow this to be negative?  Maybe a [positive integer field](https://docs.djangoproject.com/en/1.4/ref/models/fields/#positiveintegerfield) would make more sense.