CaptchaAction is used together with Captcha and CaptchaValidator
to provide the CAPTCHA feature.
By configuring the properties of CaptchaAction, you may customize the appearance of
the generated CAPTCHA images, such as the font color, the background color, etc.
Note that CaptchaAction requires either GD2 extension or ImageMagick PHP extension.
Using CAPTCHA involves the following steps:
1. Override [[\yii\web\Controller::actions()]] and register an action of class CaptchaAction with ID 'captcha'
2. In the form model, declare an attribute to store user-entered verification code, and declare the attribute
to be validated by the 'captcha' validator.
3. In the controller view, insert a Captcha widget in the form.