Email confirmation fail on multiple confirmation attempts (#8656)

If an user already confirmed his email address and tries to confirm it again it should fail.

src/Orchard.Web/Modules/Orchard.Users/Services/UserService.cs

@@ -120,7 +120,11 @@ namespace Orchard.Users.Services {
             var user = _membershipService.GetUser(username);
             if (user == null)
                 return null;
-
+            
+            if (user.As<UserPart>().EmailStatus == UserStatus.Approved) {
+                return null;
+            }
+            
             user.As<UserPart>().EmailStatus = UserStatus.Approved;
 
             return user;
@@ -195,4 +199,4 @@ namespace Orchard.Users.Services {
             return user;
         }
     }
-}
+}