Added ICurrentUser.GetId() extension method.

src/Volo.Abp.Security/Volo.Abp.Security.csproj.DotSettings

@@ -0,0 +1,2 @@
+<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+	<s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">CSharp71</s:String></wpf:ResourceDictionary>

src/Volo.Abp.Security/Volo/Abp/Users/CurrentUserExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using JetBrains.Annotations;
 
 namespace Volo.Abp.Users
@@ -22,5 +23,12 @@ namespace Volo.Abp.Users
 
             return value.To<T>();
         }
+
+        public static Guid GetId(this ICurrentUser currentUser)
+        {
+            Debug.Assert(currentUser.Id != null, "currentUser.Id != null");
+
+            return currentUser.Id.Value;
+        }
     }
 }