Add random sensor mode

There is a new flag, -random, that can be used to set up dummy toilet sensors
that change pseudo-randomly. This can be used to e.g. test the app locally.
server: Send the sensor states over WebSocket

Send the sensor states over the WebSocket on connection established.
The sensor states are sent as events, i.e. there is a message sent for every
sensor registered and it has the same format as the sensor change event msg.
main: Fix default flag value