[PM-6794] block legacy users from authN (#4088)

* block legacy users from authN

* undo change to GetDeviceFromRequest

* lint

* add feature flag

* format

* add web vault url to error message

* fix test

* format

src/Core/Services/Implementations/UserService.cs

@@ -1304,6 +1304,28 @@ public class UserService : UserManager<User>, IUserService, IDisposable
         return IdentityResult.Success;
     }
 
+    public async Task<bool> IsLegacyUser(string userId)
+    {
+        if (string.IsNullOrWhiteSpace(userId))
+        {
+            return false;
+        }
+
+        var user = await FindByIdAsync(userId);
+        if (user == null)
+        {
+            return false;
+        }
+
+        return IsLegacyUser(user);
+    }
+
+    /// <inheritdoc cref="IsLegacyUser(string)"/>
+    public static bool IsLegacyUser(User user)
+    {
+        return user.Key == null && user.MasterPassword != null && user.PrivateKey != null;
+    }
+
     private async Task<IdentityResult> ValidatePasswordInternal(User user, string password)
     {
         var errors = new List<IdentityError>();