Resolve "Multiple discussions per line in merge request diffs"

What does this MR do?

EE counterpart of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28748

Adds a possibility to create multiple discussions on a certain line of code. Also, it changes the design of a discussion on diffs.

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.
• Tested in all supported browsers

Closes #32452 (closed)