add implementations of login and findByEmail methods

src/main/java/mate/academy/service/AuthenticationService.java

@@ -1,5 +1,7 @@
 package mate.academy.service;
 
+import mate.academy.model.User;
+
 public class AuthenticationService {
     /**
      * Imagine that some user wants to login to your site.
@@ -11,6 +13,9 @@ public class AuthenticationService {
      * Return false in any other cases.
      */
     public boolean login(String email, String password) {
-        return false;
+        UserService userService = new UserService();
+        User user = userService.findByEmail(email);
+
+        return user != null && user.getPassword().equals(password);
     }
 }

src/main/java/mate/academy/service/UserService.java

@@ -15,6 +15,12 @@ public class UserService {
      * Return <code>null</code> if there is no suitable user
      */
     public User findByEmail(String email) {
+        for (User user: users) {
+            if (user.getEmail().equals(email)) {
+                return user;
+            }
+        }
+
         return null;
     }
 }