Add support for a "since" parameter to the people-updates WebSocket

[eemeli]

At the moment, the server starts sending updates on the /api/kansa/people/updates WebSocket as soon as it's operational, which e.g. kansa/admin applies to the data it received from GET /api/kansa/people. If an update happens in that interval, it's not reflected in the client state. Adding support for a ?since=[timestamp] query parameter to the WebSocket could help make sure any such updates can be sent to the client when the socket is initially opened.

[eemeli]

This is a requirement for our integration with Grenadine to work well.