[keyserver] Make all url.json files optional

Summary:
Now we won't crash due to a missing url.json file until we actually need it.

Depends on D3977

Test Plan: Make sure I can still use the keyserver without commapp_url.json or landing_url.json

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3978