Applying a template should not be allowed when thumbnail generation is enabled

Follow-up of #817 (closed)

1. Create a process
2. Check "Generate thumbnails" in the advanced settings
3. Apply a template

This puts the user in a pretty bad position: the thumbnail generation cannot be toggled anymore, but the button requires either disabling thumbnails or removing workers, so the user has to start over by deleting all the workers first, turning off thumbnails generation, then applying the template.