Fixed bug creating im_attendance_interval objects - return hash didn't include id and attendance_id fields

Rest: im_rest_error now returns the passed HTTP status code (304, 4xx, 5xx) which was ignored previously

  - Now all dates in the Gantt Editor and in other places are
    relative to the server timezone
  - The server doesn't send any time zone information to the
    JavaScript client anymore.
  - The JavaScript client doesn't send any time zone information
    to the server anymore.
  As a result, a task that starts at 9:00 am when created
  by a computer in Europe will also start at 9:00 when viewed
  from a US computer

- Fixed the system URL, which was wrong

  in able to deal with ADR (and other...) configurations where the status
  "Open" has been disabled

  No idea where a null comes from, but anyway...

- Now showing and converting lags according to the lag format
- Extended im_timesheet_task_dependencies by diff_format_id
  and created  categories for representing lag formats
- Fixed saving and loading dependencies in REST
  /intranet-rest-data-source-procs.tcl and
  project-task-tree.json.tcl

- Disabled anything except Finish-to-Start dependency in
  the GanttSchedulingController
- Still need to adapt display in GanttBarPanel

- Fixed permission issue creating Gantt tasks of a customer
  who is PM of a project