Enable/disable platform-key push delivery for an app

Push keys uploaded to the base app serve every app of the install, with no limits and no settings: an app without keys of its own sends with the base app's keys, an app that uploads its own keys sends with those, and parent/child relations between apps play no role. This switch only turns platform-key delivery off for one app (enabled: false) or back on; it can also be set when the app is created (platformPushEnabled on POST /v1/apps and /v2/apps). Accepts a user token with appSettings.update permission on the target app, or a server (B2B) token of that app or of its parent app.