Adding new styles to the new donation button.

This is for the story https://openedx.atlassian.net/browse/ECOM-414
Sub Task: https://openedx.atlassian.net/browse/ECOM-447
ECOM-414 introduces a new donations button to the Enrollment message on the dashboard. This PR styles that button to match the new charcoal style notifications.

This PR also covers:
* Hiding the donation button when DonationConfiguration is not enabled on the platform
* Modifying all copy for notifications across the dashboard

Here is a screen shot of the new notification, with the button.
![image](https://cloud.githubusercontent.com/assets/235752/4565762/d3da460e-4f22-11e4-9f91-949e432a3597.png)

Here is a screen shot of client side validation within the notification:
![image](https://cloud.githubusercontent.com/assets/235752/4565892/e53bc4bc-4f23-11e4-9038-7ab08f46682a.png)

@wedaly @rlucioni @dianakhuang Please review the code changes (probably only need one or two folks)
@frrrances @marcotuts Please take a look at my SASS and the final styling (and thanks a ton for all the help so far!)
@griffresch @andywaldrop Verify it looks as expected, and you can see it on the following sandbox: http://wedaly.m.sandbox.edx.org/dashboard

NOTE: I will take the decimal point out of the default value, so it will just be '25'. 
woops. Checked in in FF and IE at one point and forgot to double check at the end. Thanks for the heads up!