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".

gin.Default() already provides the default logging and recovery
middleware, thus the explicit inclusion is redundant. Those statements
cause duplicate logging and error reporting using the default output.