Post apiv1campaignsgenerate channel prompt

The ad text, markup and all, used only as material for writing the channel-search prompt. Same cap as everywhere else the ad text is accepted, so a text the editor considers valid can never be refused by a request-size rule on one endpoint and not another.

A ready-to-edit channel search request written on the advertiser's behalf, in the same language as the ad copy: what the ideal channels post about, who reads them, how big they should be and what language they publish in. Meant to be dropped straight into the prompt field of match-channels-by-prompt, so it is capped at 300 characters — a search box, not a brief.