Let users know they have reached the maximum number of descendants for epics

Problem

• We have a maximum of 5 nested levels for epics (that fifth epic can have many issues, which results in a total of 6 levels)
• The user has no way of knowing this until they try to add an additional epic, which currently results in a vague error message

Proposal

• Display a message of some sort when the user has reached the maximum number of descendants in the epic/issue tree