use landing base route path to construct endpoints

Summary:
we should avoid hardcoding paths like /commlanding/. instead we should get the paths from the landing_url.json file.

depends on D3413

Test Plan: flow

Reviewers: atul, palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3414