Make importance thresholds configurable per Epic

[mikron-ia]

An extension of #384, this is about making the Importance thresholds configurable per Epic. Basic design

• Config should be entered on Epic edit screen or some other configuration subscreen
• The form should have defaults visible and replaceable
• The calculations should take Epic setting first, they are absent, it should use the current defaults from Make importance thresholds configurable #384

Same settings apply to Character and Group - there is no need for distinction.