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

The submission process contains some logic that
could be either handled using default values in the
model (where possible) or abstracted into model
methods. This has the great advantage that the
individual parts can be unit-tested (which is a lot
harder to do with code in the view).

For now there is still some commented-out code for
easy reference. This code will be removed as it is
replaced by the classes above it.