[server] Factor out clientURLFromLocalURL so it can be reused in website-responders.js

Summary:
We're changing this logic because now that we're using Express routers directly, req.url gets the baseRoutePath for the router stripped out.

See the docs for req.originalUrl:

the “mounting” feature of app.use() will rewrite req.url to strip the mount point.

Depends on D3553

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/D3554