Setup basic code for login.

API/Entities/Interfaces/IHasConcurrencyToken.cs

@@ -0,0 +1,19 @@
+namespace API.Entities.Interfaces
+{
+    /// <summary>
+    /// An interface abstracting an entity that has a concurrency token.
+    /// </summary>
+    public interface IHasConcurrencyToken
+    {
+        /// <summary>
+        /// Gets the version of this row. Acts as a concurrency token.
+        /// </summary>
+        uint RowVersion { get; }
+
+        /// <summary>
+        /// Called when saving changes to this entity.
+        /// </summary>
+        void OnSavingChanges();
+
+    }
+}