Provide JQL functions to query epic relations

Before the GH 6.1 upgrade, we used links for the relations between epics and e.g. stories. These relations could be queried with JIRA or JQL Tricks.

Since GH 6.1 the epics are actually linked using the "Epic Name" custom field and searches like the following are no longer possible:

• Search for all Epics without any story
• Search for all stories related to Epics that are "In Progress"
• Search for all stories that have an Epic a certain team is working on
• ...