Add tests for RequestPublishedArticleForm

I'm not happy with this yet. The template requires all forms to be
instantiated at all times, while relying on javascript to hide them.
There should just be an if-statement checking if the form object exists.
That way, the error handling, which is now on the form level, will render
properly and everything is much shorter.

This commit contains the removal of html from 'the important' models
 or at least contains replacements for model methods such as `print_as...()`,
 etc. etc. Except for `header_as_li`, methods are not completely deleted yet.

> Fixed/Improved the nested comments and comments in reports. They were
 not shown before.

> Improved DRY principle in several templates/codes.

> Created Managers in the `submissions` app to filter i.e. "pool" submissions
 (so exclude potential author from judging on his own publication) for the
 pool view in the query; not in the template!

> More...

The select_related method reduces the database load
 by a factor of +/- 5 on the list view.

Write tests and refactor html.

The url is based off of the DOI. Because the DOI is always the same,
making more than one factory instance crashes the detail page.

UnpublishedVettedCommentaryFactory does not have this problem,
as it has an empty DOI field and a valid (random) arxiv identifier.

Also fix missing template error for modified thesis links.

The Comment model now has a option to
 append a file (pdf, png, jpg) to itself.

Start with making commentaries pages responsive.

Start transforming the prefill with Arxiv commentary request
 to a class based view using the Arxiv caller.

RequestCommentary is transformed to a class based view. Further
 moved strings to `/strings` and used django.messages middleware
 to pass messages to the user instead of context data.

Migrations involve appending default values to model fields

Basic tests to check response contains commentaires.

Write test for bug from last commit.