Have the flow/exists endpoint return the flow on success

[PGijsbers]

The PHP API returns a standard response:{"flow_exists": {"exists": true or false, "flow_id": id or -1}}. However, to retrieve the information whether or not a flow exists, we need to query the database and get just as much information as when actually returning the flow (ignoring any subflows). So returning the flow information does not generally occur much additional overhead.
It also feels like it is just another side of the same coin: you get the flow by its unique id, or by its unique combination of (name, external_version). It would make sense for me to make those endpoints behavior the same w.r.t. finding and not finding a flow.
Because of the extra network traffic (a little bit of json) and db load (in case of resolving subflows), we could also consider returning the whole flow based on a parameter, or simply have an extra endpoint that has this behavior.