all: All JSON output/input should be using `_` instead of camelCase

[aeneasr]

Unfortunately, the OAuth2 spec uses underscores to for word separation. For example, expires_at and access_token. The same applies to the client endpoint.

Some endpoints however, specifically the consent and warden endpoints, use camelCase. This leads to inconsistencies between the APIs without good reason.

We should use underscores in JSON payloads and query parameters everywhere in order for a coherent API.

[aeneasr]

Done!