[Calendar] Link calendar from Google to InNoHassle

[dantetemplar]

Describe the feature

InNoHassle may be not second, but first place to check your schedule. However, for drive this experience we need to allow users link other calendars (f.e. Google, Apple and so on) into InNoHassle calendar.

Backend is already implemented one-zero-eight/events#42 for linking any .ics url to innohassle, now we need to use this.

Suggested solution

User will click "Sources" and will be suggested to paste a link from his calendar app. Then frontend will request this ics from $events.

Additional context

No response

[dantetemplar]

Mark it as urgent because backend waited already 2 years.