Add tests for RequestPublishedArticleForm

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

New Commentary requests were not completed
 as the `requested_by` field was not properly
 saved.

Add more VetCommentary tests for the
three different possible valid form actions.

Emails sent from the vet request commentaries
are contains in templates now in the templates folder.

Logic from vet_commentary_request has been moved
to the VetCommentaryForm form. Further moved some
code to improve DRY.

Method test_form_with_duplicate_*'s second part
 gave a false-positive result as the form_data
 contained neither a DOI nor a Arxiv ID. The
 tests came back positive but actually they were
 not.

Move few constants to a seperate file away from models.
This implies minor edits in other apps relying on
these few constants.

Move form validation and saving of instances
to the form class. From here you are able to instantly
save your validated form to the database. Further
reduces the size of the views.
Form may need error handling on not having
found a Contributor before saving the form instance.

Lost order argument in query for CommentarySearchForm
results in the last commit.

Reduce the amount of logic in the `commentaries` and
`browse` views methods in the `Commentaries` app. Further,
added a return method to the `CommentarySearchForm` to help thin
out code in the view.