Add possibility to subscribe to external ics-calendar.

[mdosch]

Please add support for subscribing to external ics-calendars.
It should also be possible to subscribe to external calendars with username and password authentification.
Update interval should be configurable.

This is actually an old feature request: https://github.com/owncloudarchive/calendar/issues/344

[georgehrke]

please search for existing issue before creating new ones

[georgehrke]

#132

[toefel1]

I can understand mdosch very good - this is one absolutely feature necessary to make the ownCloudCal a No.1 choice Calendar...

[DeepDiver1975]

I can understand mdosch very good - this is one absolutely feature necessary to make the ownCloudCal a No.1 choice Calendar...

this is implemented for a long time.