Add `PtsMembership` command

Shells out to `pts` command line utility for checking members of
specified AFS group. Handles when command has a 0 exit status but
returns an error message.
Add worker to sync users that have the desk worker role

Runs every 5 minutes. Notifies `Honeybadger` if:
* `ENV['DESK_WORKERS_GROUP']` is missing
* The command failed.
* A user in the group is not found in the application database or the
  LDAP server