Add Accept header negotiation to relevant API endpoints

[tsmethurst]

This PR adds server-driven content negotiation logic to API endpoints. See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#server-driven_content_negotiation

This is done by adding a NegotiateAccept function in the api package, which can be called by handler functions that return specific content types.

In cases where the requested content-type is not available, the server will reply with code 406 and an error message:

{
  "error": "no format can be offered for requested Accept header(s) [text/html]; this endpoint offers [application/json]"
}

For now, all handler functions that need to do content-negotiation call the NegotiateAccept function manually. In future, we could look at grouping these handler functions and applying gin middleware to router groups, though this will require some fiddling.

This PR also doesn't handle redirects when a requested content-type is not available, for now it just returns the 406. In future we can look at doing redirects for specific endpoints, but this should (imo) be a separate PR.

closes #328