Improve collection of pingback statistics and add stats for enabled and disabled extensions

Currently we only get pingback stats when a user visits the Administration Console page - which is now only infrequently used (given the Admin navigation menu). We should modify pingback trigger - tentative plan:

• check cache for last time pingback sent on every non-public civicrm page load
• if 'stale' send new pingback and update cache

We should also evaluate the stats currently being sent and potentially remove some / add others. We definitely want to add stats for installed + enabled extensions (using extension key + release version).