Add floss.fund funding.json to get per-instance funding

[JonLatane]

After reviewing how https://floss.fund works and its funding.json manfiest work, I'd like to host manifests https://[jonline.io|bullcity.social|oakcity.social]/funding.json.

Add values to the ServerConfiguration to support adding payment info, etc., to funding.json, probably with the server maintainer acting as a "steward" of Jonline as open-source software.

Hosting funding.json on the server, and requiring the funding data be pointed at an Admin user, should mean we don't need to do any /.well-known/... paths or use floss.fund's wellKnown JSON values.