Prevent 'undefined index' errors when syncing calendars using iOS devices (not sure if it's specific to iOS). These happened a lot and filled the logs with useless information.
Fix calendar event organizer when using the "username mapping" feature of BackendCombined (either through the config file or the new state machine mapping). If the organizer was absent, the username used to connect the CalDAV backend with is used instead of the username as supplied by the user. At least iOS requires this username to be identical to the one used to connect to Z-Push with, otherwise it will render the event as an invite instead of an "owned" event.
This PR contains two fixes: