iCal (On The Web) support

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

Remote iCal files can't be added into Calendar without using Evolution.

**Describe the solution you'd like**

Calendar's adding menu looks derivative of Evolution's, and so _if possible_ we could port the menu.

**Existing work**

This is already implemented in Evolution. Under Evolution Accounts, Add, Calendar and **On The Web**.

**Additional context**

Here is a comparison of Calendar's adding dialog on the left, and Evolution's on the right:

The Evolution menu also allows for importing local .ical files. I wouldn't need it, but this would probably be a useful feature to some people.