Auto-import iTIP requests as pending

[mmonterroca]

Summary

• add an opt-in calendar_itip_autoimport_pending setting
• auto-save incoming iTIP REQUEST messages to the default calendar as NEEDS-ACTION/tentative when opened or previewed
• add event CSS classes from the current user's PARTSTAT so pending/tentative/declined invitations render distinctly

Notes

• no RSVP is sent by auto-import; users still respond explicitly through the existing iTIP controls
• existing events are not duplicated; newer iTIP sequence/change revisions update the saved event

Testing

• patch applies cleanly against current master/3.6.1
• deployed on a Roundcube 1.6.5 + Kolab Calendar 3.6.1 installation and validated with php -l on calendar.php/config.inc.php