Stop serving apps on herokuapp.com subdomains

[grega]

Staging and production apps on herokuapp.com subdomains should not be accessible, but currently are:

• https://editor-api-staging.herokuapp.com
• https://editor-api-production.herokuapp.com

(https://staging-editor-api.raspberrypi.org and https://editor-api.raspberrypi.org should be used instead).

The application can be configured (via config.hosts) to only respond to requests on specific (sub)domains, eg: https://github.com/RaspberryPiFoundation/experience-cs/blob/main/config/environments/production.rb#L78-L79

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Note that review app URLs also need to be explicitly set in this case eg. https://github.com/RaspberryPiFoundation/experience-cs/blob/main/config/environments/production.rb#L82

Settings for this are here:

[create-issue-branch]

Branch issues/533-Stop_serving_apps_on_herokuapp_com_subdomains created!