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.

Update all involved tests. Also extend model_form_data to
accept an argument `form_kwargs` to pass to form.

Theses that are accepted are vetted by the correct contributor.
Mails are not yet sent. Thesis is not yet deleted when refused.

This custom factory subclasses factory.Factory,
and overrides the build method, so that the model_class
gets called with a dict of fields, instead of with keyword
arguments. This is the way Django form classes require data to be
passed to __init__.

This function helps to create form data from models.
See example given.