Create newsletter API endpoint #30

This is related to #29

Create the endpoint where a new newsletter can be sent to the mailing list.

This ticket should just be the api endpoint and the necessary tests.

POST /api/v1/newsletter
Should be where the form gets submitted to send an email to everyone on the mailing list. (the implementation of this will be a separate task).
This should accept a JSON object that is something along the lines of:

{
    "content": "newsletter content here"
}

A formal spec of the endpoint will be added created later.