[re #12346] Init of saving relation between issues and attachments

In this merge request:

• Added support for saving relation (both-sided) between Issues (work_item) and files attached to issue (work_item - a.k.a artifacts)
• To implement that functionality, I needed to create new services in core package and add some methods into global repository package
• Now, the relation is saved successfuly between two items and the core package is extended by the Relationservice and WorkItemRelationService
• I also made the code cleaner than in the previous MR (I moved logic into mapper and the saving of the entities are now moved into core services)

In following MR, I will add more relations (e.g. issues vs issue etc.).

Me when John Pork (a.k.a Jakub Pavlíček) finds out I am slowly becoming Spring Boot hater.