[server] Add baseRoutePath to app URL config JSON

Summary: Just basePath (external) isn't enough – to properly detect what external URL triggered the request, we need to check if the internal URL starts with a particular baseRoutePath. We also need this in order to find/replace the internal URL so we can get the external URL.

Test Plan: This diff was tested in combination with the following diffs. On its own, it just changes a type

Reviewers: varun, atul, palys-swm

Reviewed By: varun

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3547