Edge cases for /api/auth/*

[amy-lei]

For /api/auth/register

• Check if the username already exists in the DB, otherwise therell be an internal server error
• Return the

For /api/auth/login

• Check if the username is valid before checking the password (because user.check_password(body.get('password')) results in an error if user does not exist)
• Along with the token, return the user's id, the user's list of contacts and the user's list of groups.