Broadcast: Standalone instance still uses full service URLs

[ao-david]

For testing purposes, it is helpful to be able to use a standalone instance which can simulate behaviours that are occurring in production. However, currently, all external service URLs point to the production versions.

This means that creating a broadcast on a local instance will make a newBroadcast request to the production version of oceanTV, rather than a local version.

It may be useful to keep using the behaviour, but it would also be good to be able to make requests to a local instance so that testing can be isolated from production.

[ao-david]

This can be set using the build arg