[server] Pass client URL to client in website-responders

Summary:
We didn't need this before because for the squadcal.org config, baseRoutePath was equal to basePath. But with the web.comm.app config, this is no longer true.

Depends on D3554

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/D3555