added ability to clear codes in bulk

### What is the problem / feature ?

I accidentally set all blank codes to "None" instead of a blank string. There are hundreds and it would take forever to fix them one by one.

### How did it get fixed / implemented ?

I modified the admin to allow for bulk clearing of codes.

### How can someone test / see it ?

Go into the shifts admin and drop down the action selector.

![2787](https://cloud.githubusercontent.com/assets/611731/2557820/adc38d48-b71c-11e3-9dae-cd96d07090ef.jpg)