Show warning for too many draft notes in MR review

What does this MR do and why?

• This MR add a warning to the user when an MR review contains more than 50 draft notes
• Why? A large number of draft notes (batched comments) can cause additional load on the backend when the review is submitted, see Submitting code review with 64 notes to MR can ... (#433461 - closed) ; to mitigate this risk of an overwhelmed backend, we believe that showing a warning to the reviewer (user) will make users aware that a smaller number of draft notes is recommended => #433461 (comment 1778136440)
• Why? We hope that showing the warning is a quick win / "low hanging fruit" / low-effort solution to reduce the performance impact on the backend server
• Possible addition: Allow customizing the recommend limit for draft notes in MR reviews; ATM, the number is static

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides
• Add tests for the alert

Screenshots or screen recordings

Short screencast: https://www.loom.com/share/3092e598bb9c4ff1b1f0f42b5f083547

Before	After

How to set up and validate locally

1. Create a new merge request for a project
2. Add more than 50 draft notes to the MR review

(1..51).each do |i| DraftNote.create(merge_request: MergeRequest.find(id),
    note: "Comment #{i}",
    author: User.find(1))
end

3. Go to the merge request and click and wait for the page to load
4. Click on "Finish review" on the bottom of the screen

Related to #433461 (closed)