Replace the sequence

Replaces the WHOLE sequence tree of the campaign. Always GET the campaign first, modify the steps object and send the complete tree back. The campaign must not be RUNNING (pause it first). Missing step and version _ids are generated by the server; keep existing ids on a campaign that already ran, otherwise history, statistics and the position of contacts in the sequence are lost and contacts may receive steps again. Content is not validated here: empty email versions, missing subjects or audio are only reported by POST .../start.