[Direct Edit] Add a warning on upload when server-side lock is disabled

Uploading a new blob of an ongoing Direct Edit'ed document should be made only when the user had already locked it.

But there is an option to bypass the auto-locking (disabled by default). So, when this option is enabled, we should add a warning:

Server-side document locking is disabled: you are not protected against concurrent updates.

This is mostly to help debugging future issues.