Add support for Epics

[benedeki]

Background

Many bigger features are detailed in smaller deliverables, each having their own issue. Would be nice to group them.

Feature

Figure out how to group issues from one epic

NB!

• Epics can be identified by label Epic, by having a tasklist or by other means - depending on team style
• epics usually don't have their own PR
• Epics doesn't have to be closed within the release
• Epics can have issues still open or closed in previous release
• Epics can have other epics in their tasklist

[miroslavpojer]

Very very low prio.