Server side mediabrowser

The Kickstarter is extended to generate a basic media browser, that lets editors upload generic files like images and PDFs and allows to choose one or more of these assets for `linklist` and the upcoming `reference` or `referencelist` attribute.

The media browser should be triggered in the inplace GUI when editing a `reference` or `referencelist` attribute. This functionality needs to be implemented in the related issue.

The browser opens in a modal dialog comparable to the widget or class browser. All styles are part of the application and do not rely on any CSS framework like Twitter Bootstrap.

Fixes #116