Allow user to select a delivery group during a web-based subscription flow

External services can implement web-based subscription flow (https://pushover.net/api/subscriptions#web) and use it as an user-friendly mechanism to create Pushover subscriptions and retrieve subscription keys. Users don't have to manually copy the keys around, which is great.

Unless I'm missing something, it looks like the web-based flow does not support delivery groups – the user cannot select what key (their user key or one of the delivery group keys) will be passed back to the external service. It would be great if this could be added, – we could then use the web-based subscription flow to also set up subscriptions to Pushover delivery groups.