Bookings overview: Indicate when new booking begins

This is mainly for the Burotel plugin. The goal is to show the overflow gradient indicator (for the full-day bookings we exclusively have there) only when a new booking starts instead of doing so between every day.

On the Indico side we need to add logic so the Burotel plugin can override this behavior.

Example:

"In the screenshot above it'd be a full orange bar since it's all the same booking. if it was two bookings. e.g. one starting on 20th, the gradient would only be shown between 19/20."