Add active session tracking

Adds a flag for a maximum number of user sessions, as well as an
authentication check to ensure that the user is not above this level.

MediaBrowser.Model/Users/UserPolicy.cs

@@ -92,6 +92,10 @@ namespace MediaBrowser.Model.Users
 
         public int LoginAttemptsBeforeLockout { get; set; }
 
+        public int ActiveSessionCount { get; set; }
+
+        public int MaxActiveSessions { get; set; }
+
         public bool EnablePublicSharing { get; set; }
 
         public Guid[] BlockedMediaFolders { get; set; }
@@ -144,6 +148,8 @@ namespace MediaBrowser.Model.Users
 
             LoginAttemptsBeforeLockout = -1;
 
+            MaxActiveSessions = -1;
+
             EnableAllChannels = true;
             EnabledChannels = Array.Empty<Guid>();