Dynamic feed availability

[testower]

The api should not link to unavailable content.

• The system discovery file should not link to urls that give 404
• Each gbfs discovery file should not link to files that give 404
• A gbfs file endpoint should be able to 404 even if it is available in the cache (? is it necessary)

This requires that the discovery endpoints content is populated dynamically based on available / mapped data continously.