Please add ICAL / ICS support

[dreamflasher]

Currently, it's only possible to add CalDav calendars. I would like to add a calendar from teamup.com (it's a service to combine multiple ICALs), but when trying to add https://ics.teamup.com/feed/ksukz352jp9g48zvv7/0.ics I get forbidden. Apparently an ICS via HTTP is something different that ICAL via WebDav, so I'd appreciate if you'd add ICS support.

[EDIT] Just tried with Google ICS links, and they also don't work, resulting in Error getting Calendar: ReportError at '405 Method Not Allowed…