[keyserver] Make getSquadCalURLFacts return optional

Summary:
This makes it so squadcal_url.json isn't necessary, unless getAppURLFactsFromRequestURL gets called and commapp_url.json isn't defined. That should only happen if a request is received is received from Apache that doesn't match the baseRoutePath in commapp_url.json.

Depends on D3976

Test Plan: Make sure I can still use the keyserver without squadcal_url.json

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3977