Whitelist "vscode://" as an allowed Editor protocol

Summary:
See PHI1647, which asks for "vscode://" to be a configurable protocol on hosted Phacility instances.

I made the configuration editable in D21008, but this can reasonably just come upstream too.

Test Plan: Viewed config in Config, set my editor URI to vscode://blahblah.

Differential Revision: https://secure.phabricator.com/D21009