Fix flash styling

Closes #2103

It looks like all the flashes were designed to match up with Bootstrap
classes for styling.

From http://getbootstrap.com/css/ under "Contextual classes" there are:
.active, .success, .info, .warning, .danger

But no ".notice" class. This commit changes all the instances of
flash[:notice] to flash[:info] so that Bootstrap will style them for us
using the ".info" class.

Before image:
![muted_things_before](https://cloud.githubusercontent.com/assets/5103414/5385729/154753ba-8093-11e4-9bb5-ec69c4cfd872.png)

After image:
![after_muted_submission](https://cloud.githubusercontent.com/assets/5103414/5385742/2d07e550-8093-11e4-81c9-2e061f29d918.png)