[web] [refactor] [ENG-971] rename assetUrl to assetsCacheURLPrefix

Summary:
rename, did a findall, replace using vscode tool

https://linear.app/comm/issue/ENG-971/rename-asseturl-to-assetscacheurlprefix

Test Plan: run landing project. Images load

Reviewers: atul, palys-swm, def-au1t, yayabosh

Reviewed By: atul

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

Differential Revision: https://phabricator.ashoat.com/D3617