refactor(core): backport & improve extensibility of `DiscussionListItem`#4

DavideIadeluca wants to merge 1 commit intodi/extensibility-improvementsglowingblue/flarum-framework:di/extensibility-improvementsfrom

di/discussionlistitem-extensibilityglowingblue/flarum-framework:di/discussionlistitem-extensibilityCopy head branch name to clipboard