cli/origin: Add schedule-high-priority-first-visits command

This new command in the origin group enables to schedule first
visits with high priority for origins registered by listers having
the first_visits_priority_queue attribute set.

The command ensures the visits of all origins registered by such
listers will be scheduled with high priority after the first listing
regardless if some have already been scheduled prior it.

Subsequent executions of such listers will no longer trigger visits
with high priority though, those will be scheduled by the recurrent
visits runner.

Related to #4687.