WebFinger: add subscribe_address in return data of JSON responses

I noticed that when a remote user uses the "Remote Follow" functionality (/main/ostatus), it fails with "Something went wrong". Reason is Pleroma.Web.TwitterAPI.UtilController#remote_subscribe (the latter) expects subscribe_address parameter to be present in a WebFinger.finger call, but it will be missing if the WebFinger response is in JSON, causing it to fail if the remote instance responds WebFinger requests only in JSON. This MR adds the missing subscribe_address parameter.