Allow the same iCal link to be added to multiple properties

Problem statement: Currently, Hospitable does not allow hosts to add the same iCal link to multiple properties. When attempting to add an iCal link that is already in use on another property, the system blocks the action. This is particularly challenging for hosts with multiple parent-child property setups who need to sync the same calendar to multiple parent properties.

Current workaround: Hosts must use different iCal links for each property, even when they want to share the same calendar source across multiple properties. There is no way to combine multiple parent listings into a higher-level "grandfather" property.

Impact: This prevents hosts from efficiently managing shared availability across multiple parent-child property setups, creating confusion and potential double-bookings.

Proposed solution: Allow hosts to add a single iCal link to multiple properties, enabling the same calendar to sync across multiple parent properties in separate parent-child configurations.

Unlock: Hosts would be able to sync the same external calendar to multiple parent properties, ensuring consistent availability management across complex property hierarchies.