Messages received when disconnected from ws are never received upon reconnection

[julianlam]

If a user severs the websocket connection but stays on the chat page (or modal, for Persona), any messages received in the interim are not displayed to the user upon reconnection.

This happens more frequently on mobile as mobile browsers are very strict about resource consumption when a tab is not active or phone is on standby, etc.

Current backend logic doesn't have capability to retrieve messages by timestamp, but is a fairly minimal tweak to getMessageIds in src/messaging/index.js to support.