Add Cancel button to discussion note form

What does this MR do?

Adds a cancel button to the discussion notes form.

Screenshots

Button disabled while text is empty

Button enabled while text not empty

Text clears when button is clicked

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

JS tests written for this cancel button. Perhaps a front-end feature spec might also be useful.