Click stats tracker requirements

Use case

 * compare actual usage of various features on the edit toolbar, sidebar, etc ... potentially in various variants

End-user workflow example
A given user is editing a wiki page. He/she has either the old-style toolbar or the new-style toolbar. We want to record:
 * which toolbar variant they're using
 * We at least need to handle the plain vs advanced preference. In the future we should consider bucket testing where folks might have one of several variants, in which case that info needs to be tied to their session or user ID or something.
 * which buttons they push during their edit session

Things we might also want to record? Are these useful/needed/etc?
 * whether the edit gets saved or not
 * "experience level" (anon? logged-in? sysop? tiered edit count?)
 * track undos?

API for saving
Performance characteristics?
 * Roan notes that recording every toolbar click might be prohibitive. Consider options for batching multiple clicks in a session or sampling sessions so we're not recording a firehose of info.

Tampering?