Add HUD button that displays your SSS role and allies (#1895)

* Add button that displays your SSS role and allies

* Capitalize button name

* Add cases for 0, 1 and invalid number of allies

* Make the ally syncing system saner

Content.Server/GameObjects/EntitySystems/SuspicionRoleSystem.cs

@@ -0,0 +1,51 @@
+using System.Collections.Generic;
+using Content.Server.GameObjects.Components.Suspicion;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects.Systems;
+
+namespace Content.Server.GameObjects.EntitySystems
+{
+    [UsedImplicitly]
+    public class SuspicionRoleSystem : EntitySystem
+    {
+        private readonly HashSet<SuspicionRoleComponent> _traitors = new HashSet<SuspicionRoleComponent>();
+
+        public IReadOnlyCollection<SuspicionRoleComponent> Traitors => _traitors;
+
+        public void AddTraitor(SuspicionRoleComponent role)
+        {
+            if (!_traitors.Add(role))
+            {
+                return;
+            }
+
+            foreach (var traitor in _traitors)
+            {
+                traitor.AddAlly(role);
+            }
+
+            role.SetAllies(_traitors);
+        }
+
+        public void RemoveTraitor(SuspicionRoleComponent role)
+        {
+            if (!_traitors.Remove(role))
+            {
+                return;
+            }
+
+            foreach (var traitor in _traitors)
+            {
+                traitor.RemoveAlly(role);
+            }
+
+            role.ClearAllies();
+        }
+
+        public override void Shutdown()
+        {
+            _traitors.Clear();
+            base.Shutdown();
+        }
+    }
+}