Core/Spells: implement DoT clipping

* Core/Spells: Implementation of DoT clipping.
* Core/Utilities: Backport "EnumFlags v3.0" from master.
* Core/Spells: Switch AURA_REMOVE_MODE to a flag set.

src/server/scripts/Spells/spell_druid.cpp

@@ -667,7 +667,7 @@ class spell_dru_lifebloom : public AuraScript
     void AfterRemove(AuraEffect const* aurEff, AuraEffectHandleModes /*mode*/)
     {
         // Final heal only on duration end
-        if (GetTargetApplication()->GetRemoveMode() != AURA_REMOVE_BY_EXPIRE)
+        if (!GetTargetApplication()->GetRemoveMode().HasFlag(AuraRemoveFlags::Expired))
             return;
 
         // final heal