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)
I think an important principle to think about is which settings are interrelated, and thus make sense to version together. I wouldn't want to have to edit two different models 'simultaneously' to correctly configure a feature. (I'm thinking here about the current state of the course mode changes needed for setting up verified certs).

A similar dividing line might be around what features should be able to be turned on and off individually. `ConfigurationModels` all have an enabled/disabled flag by default. If it would make sense to turn on/off something that uses a subset of the fields in a `ConfigurationModel`, then maybe those fields should be their own model, instead.