Put work_items/new page behind a feature flag

What does this MR do and why?

This MR puts the /work_items/new behind a feature flag, because we want to create work items inline instead.

The feature flag (work_items_mvc_2) already exists and is being used to allow work items work visibility to gitlab-org/plan-stage: https://docs.gitlab.com/ee/architecture/blueprints/work_items/#feature-flags

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. In the master branch, go to a work items new URL locally (for example http://localhost:3000/flightjs/Flight/-/work_items/new).
2. Switch to this branch, go to the same URL. Should now see the work item's 404 message.
3. To validate that it is back on when the feature flag is turned on, enable it through the console.

   Feature.enable(:work_items_mvc_2)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.