Zone invasion boss HP should be recalculated after the finale begins

[Terram]

This mostly applies to Ember Isle bosses, since the boss spawns either 2 or 3 phases prior to the grand finale, or is always there. The way scaling works right now, the boss takes into account the amount of players in the zone when they are spawned. Zone events tend to ramp up in player count as they progress, so you end up with the oddity of having a boss tuned for 30-40 people fighting 6 full raids and dying in like, 30 seconds. If the boss HP was recalculated as they were descending to the ground (in the case of Siccaron and Ochrin) or their shield failed (Alitha), it would probably work better. Ithkus works fine at the moment.

Also, small request, but can we change the skybox when those events happen? Sometimes I don't even know a zone event is going on because everything looks like a lush tropical jungle, like usual. If I looked up and saw that the sky was clouded with ash, or a ton of rain was coming down, it'd really help my immersion.

[JorlynEndless]

Old world zone event bosses need this as well. They die almost instantly now.