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'. 
I think I led you astray here... We have an error color variable that would be better here: $error-color