Gkeys waited on the key VALUE to decide the script had loaded. With an empty
gmaps key (any deployment that has not got one, and settings-ci.json) the
ReactiveVar stayed falsy forever, so every component mounting after the script
had already loaded — i.e. anything reached by client-side navigation — sat on
"Waiting for the gkey" and rendered an empty <div>. /subscriptions/new was
completely blank, with no error in the console: you simply could not add a zone.
Tracking "loaded" separately from the key fixes it. Found while writing the e2e
suite, which could not get past the add-zone page.