Move upload button of content browser on the right of search widgets

  instead of in the footer, where user often miss it.

This pull request is based on the real experience that many users miss the 'upload' button that was in the footer.
The goal is to make it more visible, but is also to provide a UI that matches with a 'good practice' user story : user wants to include a content into his document, first he searches it already exists in the site, if it doesn't, there he uploads it.