[FF] Showcase Enhancements v1.2.11

Added two new permissions to items:

This allows you to separate item editing from attachment editing.
Error shown if "Delete own item attachments" is No/Never

Error shown if allowed, but time limit expired

NA

• Removed the hardcoded CSS from the element and put everything in a ff_se_threadthumbnail.less template.
• Also set the default CSS background position to "center" - so everything looks a bit nicer now.