Redmine Issue Sync

The synchronisation can either take place on single project level or on system project level.

Steps:

1. determine the synchronisation target and allocation criteria
2. determine the source
3. check whether to synchronise at all (via timestamps)
4. find involved projects
5. for each project collect issues w.r.t. its allocation criteria
6. sort out double issues
7. sort out issues already synchronised
8. execute the synchronisation