graphqlbackend: Escape path in TreeEntry.url
Created by: keegancsmith
Test plan: unit and browsing locally github.com/ggilmore/q-test
Part of https://github.com/sourcegraph/sourcegraph/issues/4408
Created by: keegancsmith
Test plan: unit and browsing locally github.com/ggilmore/q-test
Part of https://github.com/sourcegraph/sourcegraph/issues/4408