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

In Submissions -> prefill_using_identifier (view).
The ArxivCaller class handles all requests and responses during the
Arxiv paper lookup. This can be used again in different places,
and is covered by a few tests (submissions.test_services).

Additionally, there are 2 tests for the views in submissions.test_views.

Also slightly pep-improves the models and views.