Allow accessing documents from admin pages

Closes #642 (closed)

Previous behaviour

Steps to reproduce:

1. Upload some type of document
2. Click the shown link to the document
3. 404 as MEDIA_URL isn't served

New behaviour

Steps to validate that it works:

1. Do same
2. Observe that the link is going to the get_document handler
3. Also observe that you're getting the correctly localised version.