client: add config option to override API origin

If the config file contains an "api-origin" key for a service, prefer it
instead of "origin". This allows users to directly point the Python
frontend to the Go backend for local development, without having to
setup a reverse proxy.

This is the Go port of [1].

While at it, use GetOrigin for symmetry with the Python code.

[1]: https://git.sr.ht/~sircmpwn/meta.sr.ht/commit/6c2ee03923fe65423a2c55e6648f555c08db827a

client: populate NodeID from service name

Drop leftover references to gql.sr.ht

client: import from gql.sr.ht