Fix URLs with file extension (.json, .turtle) ending with regex pattern (or completely disallow file extension URLs altogether)

[bertspaan]

• [http://localhost:9292/objects.json]() is allowed, while
• [http://localhost:9292/objects/1234.json]() is not.

Grape, by default, allows URLs to end with a file extension. This does not work well together with URL patterns ending with a regex pattern:

resource '/:cdk_id', requirements: { cdk_id: ::Helpers.alphanumeric_regex } do
   ...
end

Or, maybe we should not allow file extension URLs and always use the format parameter (?format=json) or the Content-Type request header.

[tomdemeyer]

We will not allow file extensions. Ugly anyway..