Core/Auras: reworked multiplicative AuraEffects calculation

- Splitted containers for flat modifiers and pct modifiers, as they now have different handling
- Amount is now multiplied only on apply; on unapply, iterate through auras and reset the counter
- Fixes many cases of rounding error due to applying/unapplying of small factors
- Allows amounts to be zeroed (ie with an AuraEffect of amount -100)
- Do a partial revert of 6dc37a9add, auras should update amounts only for items allowed (ie no more giving crit to a sword while having an axe in the other hand and being Poleaxe spec'd)
- SPELL_AURA_MOD_SCALE now scales additively, rather than multiplicatively (checked in sniffs)

Closes #18687

src/server/game/Spells/SpellEffects.cpp

@@ -3349,7 +3349,7 @@ void Spell::EffectWeaponDmg(SpellEffIndex effIndex)
             case RANGED_ATTACK: unitMod = UNIT_MOD_DAMAGE_RANGED;   break;
         }
 
-        float weapon_total_pct = m_caster->GetModifierValue(unitMod, TOTAL_PCT);
+        float weapon_total_pct = m_caster->GetPctModifierValue(unitMod, TOTAL_PCT);
         if (fixed_bonus)
             fixed_bonus = int32(fixed_bonus * weapon_total_pct);
         if (spell_bonus)