[server] add helper function to generate all route paths

Summary:
the new base path that web.comm.app will proxy to needs to be added to the express router. this diff makes that change and also deduplicates logic around generating route paths.

depends on D3414

Test Plan: flow, ran the web app and everything looked fine

Reviewers: atul, palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3433