Enable a campaign for sending, setting its status to scheduled (1).

Only a campaign whose status is draft (0) or disabled (6) can be enabled. For a one-off campaign this also queues the sending engine, so the campaign sends at its existing scheduled date; for a recurring or RSS campaign the next scheduled date is recalculated from the campaign's recurrence settings when it is re-enabled. The scheduled date itself cannot be set here - use PUT /api/3/campaigns/{id}/edit.

This endpoint reports business-rule failures in the response body rather than with an
error status: an unknown campaign, an automation ("series") campaign, a completed campaign and a
transition that is not allowed from the campaign's current status all return 200 with succeeded
set to 0. The message text is localized to the account language, so match on succeeded rather
than on the message.