Let the time of day be part of the `when` attribute of an event

As a user of the bot, I would like to be able to see the time of an event, not just the date. Usually, presentations are scheduled for 17:00, but that is not always the case.

This requires some larger changes to the backend, as the database entry for `when` is a `date`, not `datetime`. A user could always also add the date in the description (the `what` field).