Algalon: More aggressive warnings for Big Bang

[Mossy]

I can't get a warning to say when a Collapsing Star is going to die soon unfortunately, but this change will give a flash-and-shake warning when there is 15 seconds to the start of the cast of Big Bang, and another flash-and-shake warning when there is 5 seconds to the start of the cast. Obviously it's untested but I've checked and double-checked it and I'm hoping it just works.

Same instructions as for Yogg-Saron, extract the file and then replace Algalon.lua in <your Warcraft folder>\Interface\AddOns\BigWigs_Ulduar\.