Provide HTTP status 409 and conflicting field on /register endpoint

Email address and username must always be unique. Register will answer with HTTP status code 409 Conflict when a user tries to use an existing username or tries to reuse an email address to create a new account.

The endpoint will also return an json object with a field "field" providing information which attribute failed.

When the username and the email address are both reused, the field will have the value "username".

Closes #16 (closed)