Read related epic links from work item

As part of moving towards work items as the SSoT we want to read data from related epic links for WorkItems. he RelatedEpicLinks::ListService is only used on internal controllers and we don't need to migrate that service but can just delete it.

However, we still need to support and refactor the the REST API, which directly calls .related_epics on the Epic model.

We can only ensure that WorkItems are the SSoT after 17.8, when the NOT NULL constraint got introduced.