Add option that allows to skip existing tickets

[bschmalhofer]

An use case for ImportExportTicket is the merging of different OTRS/OTOBO/Znunx/KIX installations. In an ideal world this would be done in one smooth process and everything will be fine. In the real world imports are repeated because the requirements are changed. A simple example would be that more years and queues should be included. When there are already imported tickets in the new import then at least two problems can arise:

• The imported ticket was already updated and the new import should not overwrite the changed attributes
• Articles would be added again, leading to duplicate articles in the target system

Both of these cases can be alleviated by not importing tickets that already exist in the target system.