Use internal ve.track to log events

This patch rounds off change I29740fa7a by replacing calls to EventLogging's
eventLog.logEvent with calls to VisualEditor's ve.track. ve.track publishes
events by providing an interface, ve.trackRegisterHandler, which event handlers
use to subscribe to VisualEditor events. By making it the responsibility of the
web analytics framework to register itself as a handler, VisualEditor can
remain decoupled from (and indeed ignorant of) any particular event logging
implementation. This allows VisualEditor to be integrated with many different
web analytics platforms with nothing more than a bit of glue code for mapping
ve's event semantics to those of the target platform.

The practical difference that this makes is that it frees VisualEditor from
having to know about EventLogging or from having to load EventLogging
components, which means we can remove quite a lot of gnarly code. My current
plan is to migrate the code for registering and loading the 'Edit'
schema module to Extension:CoreEvents, which is also where I'll commit the
handler for VE events. (CoreEvents exists precisely to provide an organized
place for persistent but WMF-particular instrumentation.)

Once this patch is merged and deployed, the following two configuration
variables may be removed from mediawiki-config:

- $wgVisualEditorEnableSplitTest
- $wgVisualEditorEnableEventLogging

Change-Id: Idfdf692668d2adfbe029e8f0c4ff9e96c60ff741