Fix incorrect latest_id value in notifications_read request

The issue is well described here: #305 (closed).

Notifications are not correctly marked as read due to FE sending incorrect latest_id.

A small screencast demonstrating the bug: https://cl.ly/094a2fcac2b0

The reason latest_id is not always correct is that in addNewNotifications, where we set maxId by comparing notification ids, notification ids are compared as strings. So, for instance, id '9' is greater than id '25'.

As a side note, notification ids are explicitly converted to strings in parseNotification after they are fetched from BE. I haven’t looked at other parts of the code to see if there is anything that depends on them being strings, but since they are explicitly cast, I wasn’t considering changing this behavior as a part of the fix. (But please let me know if you think otherwise.)

Screencast with the bug fixed: https://cl.ly/0c437171ce03