MDL-56129 core: Set a timeout on the session cookie
This is primarily because iOS has changed something under the hood which means that only session cookies which have an expiry are passed around the OS. In order to make media playable outside of the browser (e.g. a video), we must set a session cookie timeout. Since the session timeout is configurable, this patch sets the cookie timeout to the session timeout plus a period of one week. This ensures that videos continue to work, and that the expired session message is shown on the login page, but without requiring excessively long session times.