Correct index for `user_roles` table

Had lingering indices from when the table was the join table between
`users` and `roles`.
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 or if the command failed.
Extract meaningful methods in SyncRolesWorker