[server] Use separate Express router for landing page

Summary:
This diff also makes us no longer needs server/facts/url.json. The baseRoutePath that was there was ostensibly a global one, but if we have distinct routers for each "app", we don't need a global baseRoutePath anymore.

Depends on D3550

Test Plan:

1. We tested the landing page, the SquadCal app, and the Comm app
2. We made sure we tried configs where baseRoutePath !== basePath
3. We also checked view-source to make sure things were set correctly
4. We made sure there were no errors in the browser console
5. We played around with the site, logging in and navigating and making sure things worked correctly
6. We tried loading a specific URL for a specific chat directly

Reviewers: varun, atul, palys-swm

Reviewed By: varun

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3551