Fixes #6514: advanced application SignupForm::signup() should not return User model if saving failed

Fixes #5088: new password reset token is now generated only if previous one was already used or expired

Fixes #4071: `mail` component renamed to `mailer`, `yii\log\EmailTarget::$mail` renamed to `yii\log\EmailTarget::$mailer`

Fixes #3179: removed duplicate validation rules from User model in advanced app, removed User::create()

Fixes #2458: Added missing validaton to advanced app forms, separated validation from email sending errors for contact form

- Unified way to use Alert
- Added link to restore password to login form
- Refactored restore password