Add placehold Vue app for "New Release" page

What does this MR do?

Scaffolds a basic Vue app for the "New Release" page being developed in #214244 (closed).

Feature flag

This MR introduces a new feature flag, new_release_page, which is disabled by default.

When enabled:

• The New Release button on the main Releases page will point to this new page (/-/releases/new)
• The New Release page will render successfully (just a blank page for now)

When disabled:

• The New Release button on the main Releases page will point to the New Tag page (/-/tags/new)
• The New Release page will render a 404

Screenshots

Not the most interesting screenshot 😆

Related to #214244 (closed)