Provide GET support for preview images in REST API

[Robert Jäschke]

It would be nice if one could retrieve the preview images using the REST-API. The implementation should be straightforward, the question is which URLs we use. Thus, before starting implementation, look at the URLs within the web interface and the URLs of posts and documents in the REST API and then suggest a URL schema for previews for bookmarks and documents.

[Thomas Niebler]

It is now possible to retrieve the corresponding previews for documents by adding a "?preview=LARGE" at the end of the REST request. This works analogously to the preview generation in the webapp. This will also be documented in the Wiki.