introduce altcha/captcha, fixes #4113

app/controllers/accessibility/forms.php

app/controllers/captcha.php

db/migrations/6.0.2_captcha_by_altcha.php

lib/classes/forms/Captcha.php

lib/classes/forms/CaptchaInput.php

lib/classes/forms/Form.php

lib/classes/forms/Part.php

lib/cronjobs/garbage_collector.class.php

lib/models/CaptchaChallenge.php

package-lock.json

package.json

resources/vue/base-components.js

resources/vue/components/form_inputs/CaptchaInput.vue

templates/forms/form.php