Move all ContentFinder.Get() calls to MultiplayerStatic.cs

[SirrWilliam]

Type DisableTradeModeButtonForOtherFactions probably needs a StaticConstructorOnStartup attribute, because it has a field GiftModeIcon of type Texture2D. All assets must be loaded in the main thread.
UnityEngine.StackTraceUtility:ExtractStackTrace ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Log.Warning_Patch1 (string)
Verse.StaticConstructorOnStartupUtility/<>cDisplayClass2_0:<ReportProbablyMissingAttributes>b0 (System.Type)
System.Threading.Tasks.Parallel/<>cDisplayClass33_0`2<System.Type, object>:<ForEachWorker>b0 (int)
System.Threading.Tasks.Parallel/<>cDisplayClass19_0`1<object>:<ForWorker>b1 (System.Threading.Tasks.RangeWorker&,int,bool&)
System.Threading.Tasks.TaskReplicator/Replica`1<System.Threading.Tasks.RangeWorker>:ExecuteAction (bool&)
System.Threading.Tasks.TaskReplicator/Replica:Execute ()
System.Threading.Tasks.TaskReplicator/Replica/<>c:<.ctor>b__4_0 (object)
System.Threading.Tasks.Task:InnerInvoke ()
System.Threading.Tasks.Task:Execute ()
System.Threading.Tasks.Task:ExecutionContextCallback (object)
System.Threading.ExecutionContext:RunInternal (System.Threading.ExecutionContext,System.Threading.ContextCallback,object,bool)
System.Threading.ExecutionContext:Run (System.Threading.ExecutionContext,System.Threading.ContextCallback,object,bool)
System.Threading.Tasks.Task:ExecuteWithThreadLocal (System.Threading.Tasks.Task&)
System.Threading.Tasks.Task:ExecuteEntry (bool)
System.Threading.Tasks.Task:System.Threading.IThreadPoolWorkItem.ExecuteWorkItem ()
System.Threading.ThreadPoolWorkQueue:Dispatch ()
System.Threading._ThreadPoolWaitCallback:PerformWaitCallback ()