Rename to Flags, tweak flag controls

- Display post "destructiveControls" in flag instead of custom buttons
- Make flags more versatile/extensible
- Delete associated flags when a post is deleted

Uninstall the Reports extension before installing.

extensions/flags/composer.json

@@ -1,7 +1,7 @@
 {
     "autoload": {
         "psr-4": {
-            "Flarum\\Reports\\": "src/"
+            "Flarum\\Flags\\": "src/"
         }
     },
     "scripts": {