Move most references to PKMember.Name to go through helper extepsions for privacy

PluralKit.Core/Models/ModelExtensions.cs

@@ -0,0 +1,8 @@
+namespace PluralKit.Core
+{
+    public static class ModelExtensions
+    {
+        public static string NameFor(this PKMember member, LookupContext ctx) => 
+            member.NamePrivacy.CanAccess(ctx) ? member.Name : member.DisplayName ?? member.Name;
+    }
+}