gcalbot: option to skip days with no events

Hi,
For the gcalbot, we have had a bunch of teammates ask for the option of having the "what's happening today" messages silent on days with no events.

Start by locating the gcalbot entry point and reviewing how its “what's happening today” messages are generated. Trace the no-events case and determine how an opt-in silent-days option should be configured; done means days without events produce no message while days with events continue to work.

Written by the indexing model from the issue text.