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Currently managed Chrome OS devices support remote commands issued by the admin
on CPanel. Add a command that wipes all the users off of the device when run
(equivalent to removing all the user PODs manually from the login screen).

Most changes are glue or wiring things through, main files are:
- device_command_wipe_users_job
- user_removal_manager

More details at the design doc @ go/remove-users-command

Bug: chromium:827467
Change-Id: I77a130bf4dda0757a60a141713baae27d2b5b5a2
Reviewed-on: https://chromium-review.googlesource.com/1107803Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570804}