Add destructible component threshold triggers (#3080)

* WIP changes

* Fix tests, merge conflict and trigger once behavior

* Update yml

* Change test strings to be consts

* Fix total damage types and classes triggers

* Simplify damage trigger logic, move state to Threshold

* Update outdated code and docs

* Change the name of IBehavior back to IThresholdBehavior

* Change human gibbing to trigger at 400 brute damage

* Change gibbing from brute to blunt damage

* Fix one (1) typo

* Add damage class trigger test

* Add missing nullable enable to thresholds

Content.Shared/Damage/DamageClass.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using Content.Shared.GameObjects.EntitySystems;
-using Robust.Shared.GameObjects.Systems;
 using Robust.Shared.Serialization;
 
 namespace Content.Shared.Damage