Implement certain attachments as domain objects

[AndyDaniel1]

An optimal solution for the problem discribed in #3138 would be to implement attachments as objects with their own detail pages (conceptually, attachments would become overarching objects, as publications and concepts already are). These objects would have a 1:n relationship to data packages, since a single attachment/document could be assigned to many data packages. From my perspective this is a tidy but not a simple solution as we have to deal with legacy problems.