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.

Fix bug that allows a forged post request to create a comment,
while the submission is not open for commenting.

and browse) to a Class Based View, using
the ListView class as a base.
Also adds in the permission checks in the
submission process, both in the
PrefillUsingIdentifier as in the
SubmissionCreateView. This should prevent
any unauthorized submissions.

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

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

Alter ArxivCaller to inheriting from BaseCaller
 and slightly alter the caller workflow. This
 prepares to be able to use ArxivCaller in
 for example Commentaries as well. Further
 initiate empty DOICaller which should
 be used to do similar work but with a DOI.

Prepare ArxivCaller to be used in multiple models.
 It includes the creation of a ArxivCallerMixin in the
 behavior file which should be used in all objects using
 the ArxivCaller.

Move ArxivCaller to the scipost folder, since this caller
 will be used by more apps, such as the commentaries
 apps.

Migrations involve appending default values to model fields