Add ability to receive notifications for all accounts logged into on current device.

When a user has multiple accounts on a device, if they are logged in to account A, they do not receive notifications for account B. This means that a user has to keep switching between accounts to check if someone has sent them a message on the account they are not currently accessing.

The issue provides no file, test, or entry point. Start by locating the client's account-session and notification handling for multiple logged-in accounts; done means messages on every logged-in account generate notifications without switching accounts.

Written by the indexing model from the issue text.