[FR] Subscribe to internet/dynamic calendars

[probablypablito]

As it stands there is no solution to have an external calendar (say a work schedule from an employer in the form of an ics file). While third party clients do exist such as ICSx⁵ for Android, this requires such a client to be installed on every single device the users wants their schedule on. An option to have them all sync in the Etesync DB (along with color support if possible) would be the optimal solution.