Get rid of (almost all) static html and generate form dynamically.
First try to make local user generation right (work in progress).
This commit seriously breaks the current functionality.
Puts some sensible stuff to config: local_users (no password hash yet),
allow_logins_for and admins.
Add a new button "add new user", so that several local users
can be created.
Remove the "local user has administrative rights"-setting.
Do some reasonable (slight too strict) input validation
and error handling for local user information.
If login and name are empty, just continue.
Move configuration file to backup path if some corruption
can be detected.  Also version number local/config.json.