Spawn a rift on NPC death

[MikeJeffers]

New feature for npc bosses

• A new despawnable rift should spawn at the boss's location when destroyed
• Rift should be vulnerable, and expire after sometime (similar to existing rifts but...)
• Rift should take user to fixed location on a fixed zone (like stronghold exit rifts!)
• Not a teleport target for other random-rifts (like stronghold exit)
• Not a Rift for TAP deployment (like stronghold exit)

What for?
New entrance mechanism for "mini" strongholds. Only accessed by defeating entrance NPC boss first.