Enrich notification payload.

[dkorolev]

@mzhurovich : Just thought, whether it's useful to pass raw UID to the client - it needs user name to display and when we allow to change it, we should send it as a plain text from the server side.

Re. struct ResponseNotification in schema.h.

[dkorolev]

Also @mzhurovich : If we think about future grouping of notifications, maybe it's worth to switch from string to vector for the fields above?