⚡️ Do not permit to create receiver if it already exist in data

[anthonypillot]

Today, if I add a new receiver with the dedicated endpoint (POST on /receivers), there is no verification if the receiver already exists. So, to avoid spam, we need to check if the receiver already exists.

So, a check have to be done on receivers, based on the email: if the targeting email already exist in data, then respond with a 409 Conflict.