Reflect maintenance window in API responses #958
Description
Context
Scheduled maintenances are currently only reflected on the UI. API users don't know about a scheduled maintenance and all endpoints are still openly accessible.
This is not ideal, as these users will encounter errors with quoting or registration of ramp services. Instead, the API should return a respective error message that describes the message and rejects usage.
TODO
Change the quote and ramp endpoints to return HTTP 503 errors with a message indicating the title and description from the scheduled maintenance table of the database.