Bootstrap 'img-responsive' behavior was opt-in for a reason but commit
https://github.com/odoo/odoo/commit/f79b00c3b71e02f73537cb17285a6902fc0906b5
tried to defy that anyway. This would break existing client databases
... or at least our odoo.com.

This commit adds the img-responsive class for *new* *editor* images
(still with inline-block display), preventing breaking libraries or
custom (prod) images.