Add instant comments support

Related EE Merge Request https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1823

What does this MR do?

Adds support for instantly creating, editing and removing comments from Issues and MRs.

Why was this MR needed?

Currently comments are updated only after network response is received, having comments posted instantly while network communication is happening in the background gives user a perception of better performance.

Screenshots

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Create Discussion

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27614 (closed)