Include or exclude one prompt from the Ads surfaces

Flags a single prompt. Excluded prompts are filtered out of every Ads response, so the Ads pages stop counting them entirely rather than showing them at zero. A prompt with no flag is included, which is what keeps prompts created before the flag existed on the Ads pages. Takes only the one prompt id, so a caller never has to send a prompt list it may have loaded before someone else edited it. This is not a transaction: the topic is read and written back, so an edit landing in between can still be lost.