[js] simplified newsletter

[wisnie]

• add event handler on input and click, if the input has valid email address, the button should become active,
• display toast with information that email address was valid

[wisnie]

resources:

• https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener,
• https://developer.mozilla.org/en-US/docs/Web/API/Document/createElement,
• https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent,
• https://developer.mozilla.org/en-US/docs/Web/API/Node/appendChild