This commit adds support to send encrypted push notifications
to devices registered to receive encrypted notifications.
URL: `POST /api/v1/remotes/push/e2ee/notify`
payload: `realm_uuid` and `device_id_to_encrypted_data`
The POST request needs to be authenticated with the server’s
API key.
Note: For Zulip Cloud, a background fact about the push bouncer is
that it runs on the same server and database as the main application;
it’s not a separate service. So, as an optimization we directly call
'send_e2ee_push_notifications' function and skip the HTTP request.
APNs apparently treats its tokens case-insensitively; FCM does not.
Adjust the `unique_together` to instead be separate partial
constraints, keyed on the `kind` of the PushDeviceToken.
This commit adds a `PushDevice` model where each row
corresponds to an account on an install of the app
that has attempted to register with the bouncer to
receive mobile push notifications.
This is the core server table storing registrations
that are potentially registered with the mobile push
notifications bouncer service.
