Adding new styles for dashboard notifications.

This covers the requirements for https://openedx.atlassian.net/browse/ECOM-422

This covers unifying the styles for all Dashboard Notifications, and making sure they are properly designed for a11y. Covers:

* Re-verification messages
* Activate Account Messages
* Third Party Authentication Errors
* New Enrollment Messages

Screen shot:
![image](https://cloud.githubusercontent.com/assets/235752/4528646/a63f2484-4d70-11e4-860f-b0220056381b.png)


Putting this up on steve.m.sandbox.edx.org  (ETA 12:15pm 10/6)

@wedaly @rlucioni @dianakhuang  If one or two of you could review the changes, let me know. Mostly template and SASS changes. (Also @dianakhuang note this involves the re-verification messages in case there is some caveat I'm not considering)
@marcotuts If you could review the SASS I'd appreciate it! @andywaldrop the sandbox will be up soon to view it.
@griffresch The sandbox will have this up soon for you to review. I may need to tweak it based on the review.
The urgency-low styles clashed a bit with the dark background, but I can take a look and see if they work well after all the other changes your suggest.