Use prefetched discussion data using startup call

What does this MR do?

• Add startup_mock_util.js containing a class StartupMock used to mock window.gl.startup_calls[<path>].fetchCall.
• Make notes_app.vue to use prefetched discussion data when startup.js call is available.

What is startup.js #220511 (closed)?

Data fetching (via calling API endpoints) usually happens during Vue app's mounting phase. To start fetching the necessary data immediately on page load, we're including a small JS script that runs ahead of any Vue app- this concept is startup.js.

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Issue #247959 (closed)