Image upload URLs returned in JSON and ignore failed moments

@leedenison please review these changes.

Image upload URls are returned in JSON {"url": "https://someuploadurl"}. The change keeps the API responses more consistent across API calls and simplifies iOS client code. This change will require corresponding updates from iOS (in review) and Android (not started).

Also, failed moment writing is ignored. This allows the app to run on a local development server, even when the app activity requests fail.