Converted the Calendar URL string to a read-only text input containing
the calendar URL. Pressing the "Copy URL" button will copy the calendar
URL to the clipboard.
Fixed Behat tests as well for the export feature. With the calendar URL
in a text input that clips the whole URL, the `preset_what` URL
parameter cannot be seen by the "I should see" step. Worked around
this by checking that the actual value attribute of the calendar URL
text input contains the preset_what parameter.

Plus: Deprecate the calendarurl string. It will be replaced by a new
lang string that does not contain the actual calendar URL.