Support custom carddav locations

[johnjaylward]

I have 2 items I'm looking for:

1. Being able to use my own NextCloud server "Out of the box" ($prefs['_GLOBAL']['hide_preferences'] = false;)
2. Disabling possibly slow connections with the preset card-dav built into mail-in-a-box ('rediscover_mode' => 'none', setting this fixed my issue here: https://github.com/roundcube/roundcubemail/issues/8533 )

I think if the out of the box carddav config was changed to look more like this, other PR / Issues may not be needed:

<?php
/* Do not edit. Written by Mail-in-a-Box. Regenerated on updates. */
$prefs['_GLOBAL']['hide_preferences'] = false; // changed from the default install
$prefs['_GLOBAL']['suppress_version_warning'] = true;
$prefs['ownCloud'] = array(
         'name'         =>  'ownCloud',
         'username'     =>  '%u', // login username
         'password'     =>  '%p', // login password
         'url'          =>  '<snip>',
         'active'       =>  true,
         'readonly'     =>  false,
         'refresh_time' => '02:00:00',
         'rediscover_mode' => 'none', // added to improve login performance 
         'fixed'        =>  array('username','password'),
         'preemptive_auth' => '1',
         'hide'        =>  false,
);
?>

1. I don't really see a benefit of disallowing users to configure their own CardDav settings. Is this something you would accept a PR for, or is there a reason to have it disabled?
2. When setting the "rediscover_mode" to "none", my logins went from 1-minute delay to instant. It seems that the feature has some major performance issues.