[server] Pass req.originalUrl instead of req.url to getAppURLFactsFromRequestURL

Summary:
Following our changes to use Express routers directly, we realized that they are stripping the baseRoutePath out.

See the docs for req.originalUrl:

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

Depends on D3555

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/D3556