MDL-29663 forum: Option to allow Forced for read tracking.

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.
MDL-29663 forum: Replace constant FORUM_TRACKING_ON with FORUM_TRACKING_FORCED

Optional commit for MDL-29663 that renames the FORUM_TRACKING_ON constant
to FORUM_TRACKING_FORCED to allow easier reading.