Add support for first-class payment/subscribe options outside of LBRY

[kauffj]

If creators want to link PayPal, Patreon, SubscribeStar, etc., we should help facilitate this in a way stronger than the custom markup in the about section.

(This ticket needs to be spec'ed before work; this is just an idea.)

[lyoshenka]

I'd suggest adding a new section on channel called something like social or services or external. This section would be key-value pairs where key is the service and value is their id at that service. we can start with a few services we support and expand from there. the id would be a string, so we could be pretty flexible in what we put in there