Add ability to cancel a design discussion

What does this MR do?

This MR adds 2 buttons to design discussion comment form: Comment and Cancel. When clicking Comment, the comment is saved as a discussion note. After clicking Cancel, if there is no text in the form, the form closes. If there was any text in the form, we show the confirmation modal to the user, asking to confirm closing the form.

Screenshots

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related issue: #37420 (closed)