Add ability to log analytic events using EventLogging

This change adds a '$wgVisualEditorEnableEventLogging' configuration
variable, accessible client-side as
'wgVisualEditorConfig.enableEventLogging'.

When true, the 'schema.Edit' ResourceLoader module will be loaded on the
page, and calls to ViewPageTarget.logEvent will forward to
mw.eventLog#logEvent.

When false, the module is not loaded, and ViewPageTarget.logEvent is a
no-op.

The change also adds a few calls to ViewPageTarget.logEvent to log certain
editor actions, documented in https://meta.wikimedia.org/wiki/Schema:Edit.

Change-Id: Iccd171d7cde15b0302d1b4c292bcbcc2a4b337ef
Merge "Add ability to log analytic events using EventLogging"