Allow accessing documents from admin pages

Thom Wiggers
2018-07-04T19:24:27+02:00

Closes #642

#### 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.