Design discussion: admin alert styling update

[Ryan Allen]

Up for discussion - https://github.com/django/django/pull/1715

The alert messages in the Django admin do not abide by the UX philosophy "Don't Make Me Think". All messages whether success, warning, or error, have a background color of yellow. This pull would change the bg-color of successes to green and errors to a light-red with red border. Also, the css color choice of red (#F00) would be replaced by shades friendlier to the eye, around #C11.

Current design: http://imgur.com/a/XMYXT

Proposed changes: http://imgur.com/a/rGBQY