Implement POST /v1/users Endpoint

[zxkevin]

As a content creator, I want a page with my user info so that all who visit the webpage can view see my info.

Tasks:

• [x] Create controller for saving users
• [x] Create the route for POST /v1/users

Acceptance Criteria:

• When a POST request is made against the POST /v1/users endpoint with a JSON request body with a first name, last name and username, the resource is created, and persisted to the database. A response with the location header set with the relative URL of the created user with a success status code is returned to the client
• When a POST request body has a blank or missing username, first name or last name an error will be returned to the client in the response.
• When a POST request body has a username that already exists an error will be returned to the client in the response.
• When an invalid POST request is made against the /v1/users endpoint an error code is returned. (In the 400-499 HTTP response code.)

[sjiang1]

Add an acceptance criterion:

• if it succeeds, the first name, last name, username, and profile picture should be stored in a database
• the username for each user should be unique (the request will be denied if the new username already exists)

[zxkevin]

Added the ACs