Write tests and refactor html.

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

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.