Multiple group assignees in epic

• Refer to epic for context.
• We want epics to be handled/worked on at the abstraction level of a "team". For the purpose of simplicity/iteration, we want to reuse groups to represent teams.
• We want to allow multiple groups to work on epics as a first step to given that use case flexibility. For example a UX team, a BE team, and a FE team may all be working toward an epic. We want to allow that.
• So the feature here is assign multiple groups to an epic.
• We purposely do not want to allow individuals assigned to an epic, in order to simplify the feature, and also for future issues outlined in the epic.
• The design would probably use the sidebar of an epic, and be similar to issue assignees.