User preview_image_link behavior everywhere instead of preview_image for all content types

[tisto]

StR

• as admin
• go to http://localhost:3000/about/edit
• -> preview_image should allow existing images

ToDo

• make sure all content objects (except image) use the prewie_image_link behavior and not the standard preview_image behavior
• make sure all content objects use the "proper" widget that shows an image preview

[tisto]

@robgietema I recall that you worked on this in the DLR project for the "Blog Post" content type. I recall that @davisagli and/or @iFlameing asked me to properly specify the widget we are looking for. Do you have any pointers for us? cc @steffenri

[tisto]

PR: #23