Move Submission recording away from views

[julen]

The translation submission view (pootle_store.views.submit) currently contains logic to record submissions and this should ideally be done in the business logic layer in models — this is already the case for other actions which record submissions, such as suggestion handling and toggling quality checks.

Having the logic in models would be more natural and would make testing easier, as it wouldn't be necessary to write test code against a view (with all the required wiring) but rather directly against the model. View code would also become simpler.

This is to some extent also related to #3895 and splitting submission types.