Add a pre-/post-edit anonymous signup invitation with bucketing

When an anonymous user clicks "Edit" or "Edit source" they are
presented with a guider that allows them to either sign up before
editing or to continue editing anonymously.

When an anonymous user edits a page they are presented with a guider
that invites them to sign up.

Additionally, log:

* when the user is shown either guider
* when the user clicks "Edit", "Edit source", or "Create account"

Remove obsolete server-side bucketing logic, but keep
$wgGettingStartedRunTest and use that for gating the feature off by
default.

Instead, use client-side bucketing, into 'pre-edit' (CTA before
edit), 'post-edit' (after edit), and 'control' (no CTA).

Co-Author: Matthew Flaschen <mflaschen@wikimedia.org>
Change-Id: Ie5ff66140a8dcff48b5cdcbed50bddc67d4a1ffc