Exclude Lead Finder Segments

Take segments out of a RUNNING lead-finder experiment's answer.

Two modes, and the difference is whether the data survives:

• default — the segments stay on the page with their own numbers and leave the totals (the same treatment a frozen segment gets). Reversible: post an empty list to put them back.
• drop: true — their enrolment and lead assignments are deleted, and they disappear from the page entirely. Not reversible.

Why this is not DELETE …/enrol: that one wipes the whole experiment and refuses to run on a live card. Taking a few segments out of a live one is a different operation with a real use — a test account whose budget got zeroed leaves a cohort that can never send, and its zeros should not be averaged into the answer while its found numbers stay worth reading.