[server] Rename clientURLFromLocalURL to clientPathFromRouterPath

Summary:
We want to be clear that the input is a path relative to a specific Express router, and the outside is a path that the client can understand.

Response to @varun's feedback on D3554.

Test Plan: Flow

Reviewers: varun

Reviewed By: varun

Subscribers: palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh, varun

Differential Revision: https://phabricator.ashoat.com/D3580