Cog API audit

[mpeck]

There are a few inconsistencies with Cog's api. For example, when returning an error, some resources return json with an errors key and a list of errors, some return an error key and a single error, and yet others return an errors key with a single error. We also have endpoint that return data inconsistently: POST /v1/groups/<id>/users returns a group, POST /v1/groups/<id>/roles returns the list of roles.

We need to do an audit to clean up the api and discuss potential improvements.