Time zone of JOC should be configurable

Currently JOC is running with the timezone of its host. It should be possible to configure the timezone.

• JOC provides a selectbox with configure timezones (e.g. Berlin, Tokyo, Buenos Aires)
  The content of the selectbox can customized in the .config/operations_gui/custom.js (see https://kb.sos-berlin.com/x/koM3)
  For example:
  _timezones[ 'Berlin' ] = 'Europe/Berlin';
  _timezones[ 'Tokyo' ] = 'Asia/Tokyo';