Add JavaScript-driven EventLogging for blog

Generating the event from PHP and including it in the HTML output is
problematic because the HTML gets cached and we get stale events. This
change re-instruments the blog using JavaScript, consistent with on-wiki
usage of EventLogging.

'jquery.json.js' provides a JSON fallback for browsers without native
JSON support. It is included by default with MediaWiki core.

Change-Id: Ic0a4a4bdf6f2f93ad10ba2b51cf3d510bf46f873
Merge "Add JavaScript-driven EventLogging for blog"