[5/n] [landing] move preloadassets to hook

Summary: Move preload assets effect to a hook

Test Plan: look in developer console and see assets are being preloaded

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2555