Redmine Issue Sync

The Synchronisation model should store all the meta data of a project based synchronisation:

• settings
• user_id
• receiver_project_id
• created_at
• updated_at

Settings need to be moved into a separate table since they won't change with each synchronisation. They should consider:

• synchronisable => will be checked before adding a button
• system_object => will collect the issues of child projects too
• trackers => will serve as filter to define a subset of issues
• custom_field_enumerations => will serve as filter to define a subset of issues
• status_for_outdated_issues => changes the status of an outdated issue; the issue does not belong to the synchronisation subset anymore