Use 409 (Conflict) response for entity clashes

## Background
Currently when an entity with a specific name already exists, if you try to create a new one with the same name you get a `400 (Bad Request)` response and a validation entity explaining the clash in the response body.

## Feature
We can be more specific in our response codes if we use `409 (Conflict)`.

## Proposed Solution
1. Change the response code to `409`
1. Keep the response body unchanged