Whenever possible, we no longer rely on the FileSaver.js library to
download zip archives. Instead, we do a simple GET on the resource by
changing the window location.
- handle Web calendars that require authentication;
- properly save the "reload on login" option;
- reload Web calendars when clicking on the reload button on top of the
events/tasks list;
- properly activate the checkbox of new calendars (as they are enabled
by default).
Fixes#3326
