feat: Implement user addition in StubUserManager

knockoutwhistweb/app/logic/user/impl/StubUserManager.scala

@@ -34,7 +34,14 @@ class StubUserManager @Inject()(config: Config) extends UserManager {
   )
 
   override def addUser(name: String, password: String): Boolean = {
-    throw new NotImplementedError("StubUserManager.addUser is not implemented")
+    val newUser = User(
+      internalId = user.size.toLong + 1,
+      id = java.util.UUID.randomUUID(),
+      name = name,
+      passwordHash = UserHash.hashPW(password)
+    )
+    user(name) = newUser
+    true
   }
 
   override def addOpenIDUser(name: String, userInfo: OpenIDUserInfo): Boolean = {