Improve validation for user creation

Since the auth and cache layer refactoring, the permission is no longer necessary for account registration.

Always check that

• loginId and authProvider properties are set,
• the user does not already exist.

If a user already exists, the response status code is now 400 instead of 403.

These changes affect user creation through registration and from the admin area.

Related: !345 (merged), arsnova-webclient!1275 (merged)
Closes: #139 (closed)