Starting a new discussion only on line without Draft note created on it

What does this MR do?

CE counterpart for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14569

With this MR the Start new discussion... reply placeholder is hidden if we have a draft discussion on this line

Please check the motivation behind this MR on https://gitlab.com/gitlab-org/gitlab-ee/issues/12632

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

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