Filter flash to only display info, success and error messages.

Avoid displaying [flash commands](https://github.com/moneyadviceservice/public_website/blob/master/app/controllers/commands/command_helper.rb) set by the public website.

![screen shot 2014-07-16 at 15 11 25](https://cloud.githubusercontent.com/assets/966819/3601852/67ee4328-0d05-11e4-9612-5327f26fb68b.png)

@amansinghb good catch!