Option to convert client timestamps to the local time zone

[nblumhardt]

Some Serilog sinks, such as Console(), show timestamps in whatever time zone they're specified in. This makes logs from the client look inconsistent with logs from the server, since Serilog.Sinks.BrowserConsole sends events with UTC timestamps.

(There's no issue with the timestamps themselves - it's just a display problem.)

We might need to add options.ConvertToServerTimeZone or similar to deal with this.