Customize default calendars and auto-create on remote login

[narcisgarcia]

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

A named "personal" calendar is only created when user already logged in through web interface, then some subscribers see no calendar and some other see 1 with CalDAV client software.

Describe the solution you'd like

All defaults for "Calendar" extension should be configurable by site administration: What calendars exist for all accounts by default and/or which of them are provided to CalDAV by default.
Those calendars should be created as soon as the user signs in and before providing calendars list.

Describe alternatives you've considered

Option to select only 1 calendar (by default) to be provided to user's CalDAV subscription, and customize its name.

Additional context

In "External user authentication" scenario some users acceed DAV services by using only their IMAP credentials.