WIP: Add new route for New Release page: `/-/releases/new`

What does this MR do?

Adds a new route - /-/releases/new - as a placeholder for the New Release page being developed as part of #214244 (closed).

The page is empty (other than the page title) and is hidden behind a release_new_page feature flag (which is disabled by default).

Screenshots

TODO

Related to #214244 (closed)