APIv4 GET /users/{user_id}/posts/flagged (#5984)

* APIv4 GET /users/{user_id}/posts/flagged

* change permission check

i18n/en.json

@@ -3703,6 +3703,14 @@
     "id": "model.client.create_emoji.writer.app_error",
     "translation": "Unable to write request"
   },
+  {
+    "id": "model.client.get_flagged_posts_in_channel.missing_parameter.app_error",
+    "translation": "Missing channel parameter"
+  },
+  {
+    "id": "model.client.get_flagged_posts_in_team.missing_parameter.app_error",
+    "translation": "Missing team parameter"
+  },
   {
     "id": "model.client.login.app_error",
     "translation": "Authentication tokens didn't match"