Sharedcontent: avoid overwriting the slug in new translations
Sharedcontent: avoid overwriting the slug in new translations

Also make the slug field less prominent,
as it's a developer feature mainly.

edoburu/django-fluent-contents,edoburu/django-fluent-contents,pombredanne/django-fluent-contents,django-fluent/django-fluent-contents,django-fluent/django-fluent-contents,pombredanne/django-fluent-contents,ixc/django-fluent-contents,jpotterm/django-fluent-contents,django-fluent/django-fluent-contents,jpotterm/django-fluent-contents,edoburu/django-fluent-contents,ixc/django-fluent-contents,pombredanne/django-fluent-contents,ixc/django-fluent-contents,jpotterm/django-fluent-contents