WIP: Let automatic registration of DOIs be optional.

[gamboz]

Add a setting to make the automatic registrations of DOIs at acceptance be optional, while still allowing the manual registration.

Rationale: ATM that the same setting use_crossref is used

• for automatic registration at acceptance Article.accept_article()
• for manual registration after publication identifiers.views.issue_doi()

so that it's not possible to disable automatic registration at acceptance while retaining the possibility of manual registration at publication (for instance).