Content item checkboxes

After sharing files, the checkboxes under the content items should be unchecked. When a user wants to share/delete/... another item, he needs to uncheck the items' checkboxes first.

![screen shot 2013-10-23 at 11 06 04](https://f.cloud.github.com/assets/2194396/1388957/d80c0634-3bca-11e3-8f23-baf514a1357f.png)