add ability to get user based on username or email address

Oqtane.Server/Managers/Interfaces/IUserManager.cs

@@ -7,6 +7,7 @@ namespace Oqtane.Managers
     {
         User GetUser(int userid, int siteid);
         User GetUser(string username, int siteid);
+        User GetUser(string username, string email, int siteid);
         Task<User> AddUser(User user);
         Task<User> UpdateUser(User user);
         Task DeleteUser(int userid, int siteid);

Oqtane.Server/Managers/UserManager.cs

@@ -51,7 +51,12 @@ namespace Oqtane.Managers
 
         public User GetUser(string username, int siteid)
         {
-            User user = _users.GetUser(username);
+            return GetUser(username, "", siteid);
+        }
+
+        public User GetUser(string username, string email, int siteid)
+        {
+            User user = _users.GetUser(username, email);
             if (user != null)
             {
                 user.SiteId = siteid;