unread_ops: Add function to mark PM threads as read.

This will be used recent topics to mark PM rows as read.

static/js/unread_ops.js

@@ -227,3 +227,11 @@ export function mark_topic_as_read(stream_id, topic, cont) {
         success: cont,
     });
 }
+
+export function mark_pm_as_read(user_ids_string) {
+    // user_ids_string is a stringified list of user ids which are
+    // participants in the conversation other than the current
+    // user. Eg: "123,124" or "123"
+    const unread_msg_ids = unread.get_msg_ids_for_user_ids_string(user_ids_string);
+    message_flags.mark_as_read(unread_msg_ids);
+}