Fix flash alerts

I noticed the flash messages were looking a little funny on the sign up page:

![screenshot 2014-12-12 10 46 57](https://cloud.githubusercontent.com/assets/816517/5414481/53a10e52-81ed-11e4-9ba2-73ab0014f271.png)

So I updated the stylesheet to include the styling that was already in there for flash messages:

![screenshot 2014-12-12 10 50 31](https://cloud.githubusercontent.com/assets/816517/5414488/67ea0954-81ed-11e4-9aa6-5f2cd2a86ddd.png)

I also saw that `#flash_notice` was defined twice with the exact same styles in the same file, so I removed the duplicate code.