Commit for MDL-29663 that renames the FORUM_TRACKING_ON constant
to FORUM_TRACKING_FORCED to allow easier and more consistant reading.

FORUM_TRACKING_ON retained but marked depreciated.

Converts the use of Read Tracking On to Forced. Adds new admin setting
forum_allowforcedreadtracking. If set, Forced will be available when
adding or editing forums. In this state the forced forum overrides the
user preference of off.

With forum_allowforcedreadtracking not set (default) behavior is the
same as now, except forums previously set in the On state are treated
as Optional.

The side effect of forum_allowforcedreadtracking being on, is that users
cannot speed up the loading of course pages with lots of forums by
setting their personal forum read tracking option to off.

Rajesh Taneja authored Aug 19, 2013 and Damyon Wiese committed Sep 02, 2013

This change includes:
* Added user_updated event, replacing old event
* Added user_created event, replacing old event
* Added user_deleted event, replacing old event
* Added user_loggedout event, replacing old event
* Added user_enrolment_created event, replacing old event
* Added user_enrolment_deleted event, replacing old event
* Added user_enrolment_updated event, replacing old event