workflows: Holding Pen files and styles

* Serve files added in submissions in the detailed view of an object.
* Add base template for workflow styles (title, description of workflow)
* Add an arXiv link if arXiv ID
Good point. In fact there is already a `get_file()` function in `deposit.views.deposit` that uses `current_user` as a filter which will not allow anyone but the submitter to access the file. That's a bit too restrictive. 

We could address this problem in deposit directly by developing a bit the `get_file` to (a) not filter by the user or (b) use authorizations to control access to submitted files.