Add a handler for multi_static_select input

The documentation of the multi_static_select input block wasn't clear
that it would trigger a call to an event handler, but apparently it does.

We don't currently impose any restrictions on what a user selects,
(any combination of rounds is valid), so we simply return a lamba_ok
value for any selection. This prevents the user from seeing a warning
sign in the dialog (where hovering over it would say that the callback
returned 502).