Refactor User Manager

[splitbrain]

Currently the user manager does the same thing (create, edit, delete users) three times:

• in a graphical user interface (in admin.php)
• via the command line (in cli.php)
• via the remote API (in remote.php)

The actual user management mechanisms should be moved out of these files into a separate thing and be reused by all three. This could either be a traditional helper component, or a namespaced class.