Bugfix: double selection of categories in user preferences 

Prevent user from selecting the same category twice for different notification levels in their preferences. Before this patch this was possible, legit and not prevented:

![screen shot 2014-04-09 at 16 39 32](https://cloud.githubusercontent.com/assets/40496/2656731/dd215d5a-bff4-11e3-9937-2253afe2acf3.png)

With this patch, the UI prevent double selection. You need to remove the item from the other list first otherwise it doesn't show in the autocomplete. (Though the backend would still accept double selection).