Add support for multiple user-accounts when self-hosting.

Allow self-host admins to create more user accounts via `transfer.zip-web/create-account.sh`.

Currently this is what happens:
```
/opt/transfer.zip/transfer.zip-web$ sudo bash create-account.sh
Email: example@example.com
Password: 
Web Server Port: 9001
{"success":false,"message":"Signup not allowed: users already exist in self-hosted mode"}
```