save_code_now: Add Web API endpoint to get info about a single request

It can be useful to follow up on a single Save Code Now request through
the Web API.

To do so, add a new Web API endpoint /api/1/origin/save/(request_id)/
to get info about a request using its unique identifier.

Add request identifier and its info URL in the JSON data returned by the
/api/1/origin/save/(visit_type)/url/(origin_url)/ endpoint and the save
origin webhook ones.