Resolve "Follow-up from "Refactor discussions/notes code""

What does this MR do?

Updates contents of app/assets/javascripts/notes/ to handle context (like issue, merge_request or epic) as provided from app root element for initializing notes app in Issue, Merge Request or Epic pages.

Why was this MR needed?

This MR reduces context-specific checks in notesapp.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • [ ] Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #44939 (closed)