FEATURE: Silence Close Notifications User Setting (#26072)

This change creates a user setting that they can toggle if
they don't want to receive unread notifications when someone closes a
topic they have read and are watching/tracking it.

spec/requests/api/schemas/json/user_get_response.json

@@ -794,6 +794,9 @@
             },
             "seen_popups": {
               "type": ["array", "null"]
+            },
+            "topics_unread_when_closed": {
+              "type": "boolean"
             }
           },
           "required": [
@@ -828,7 +831,8 @@
             "text_size_seq",
             "title_count_mode",
             "timezone",
-            "skip_new_user_tips"
+            "skip_new_user_tips",
+            "topics_unread_when_closed"
           ]
         }
       },