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.
We should avoid using direct hex values in all of our styling. This should use one of the dark edX gray variables from the LMS variables.scss file.