[server] initial commit to support new app URL

Summary: per https://linear.app/comm/issue/ENG-660/make-web-app-accessible-via-commapp, we need a different JSON file for the new web app URL. this diff adds some functions to fetch the appropriate data from files and introduces a small rename.

Test Plan: ran flow

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3406