Add user management

[gersteba]

Currently there is just one web user 'admin', for which you can set a password.
It would be great to have opportunity to configure at least one non-admin user, too.
This non-admin user should have less rights, than the admin user, of course.
i. e. he can just have the main menu view with the 'Toggle' button and without 'Configuration', 'Firmware Upgrade' and 'Console'.
Or maybe it even could be configured, which rights the non-admin user does have.