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.
This linked file has the HTML structure to use the notification styles I have set up in _system-feedback.scss and it would be great to use that same structure for all the dashboard and course notifications. The HTML skeleton is like this: 

```
<div class="wrapper-msg [optional urgency class]">
  <div class="msg [optional msg-type]">
      <div class="msg-content">
        <h2 class="title">${_("This is the Message Title Text")}</h2>
        <div class="copy">
          <p>
            ${_("This is the longer message text.")}
          </p>
        </div>
      </div>
    </div>
  </div>
```

You should be able to place this inside the dashboard-banner div and it should then use the set of styles. "urgency-high" makes the background black and the text white, "urgency-low" make the background a light grey with black text.