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)
Thanks @cpennington that makes sense. I'm inclined to leave this as-is, then. I like the grouping of small tweaks to the dashboard rendering and behavior to be in a single model. The only other thing I can think of would be "DashboardMessageConfiguration" but enabling/disabling that seems a little odd, and it may be a pain to have to find that, as well as other separate dashboard configuration settings.