This allows an availability plugin to indicate that a selected condition
should be considered "private", and therefore should never be shown to
users who don't satisfy the criteria. The availability_groups plugin
uses this to protect visibility of groups that are not visible to
non-members.
If there is invalid data in the availability condition (note: this
should not happen, but can do in unusual situations, such as
specifying a date after 2038 in 32-bit PHP), this should not
leave the course so broken that users cannot edit the activity and
update it.
This commit defines the new /availability root folder, with
/availability/classes, /availability/tests, and
/availability/condition where the condition plugins will live.
Condition plugin prefix is availability_, e.g. availability_date.
Rationale for this organisation:
1. I was originally going to put this in /lib/availability but
it has been pointed out that putting even more junk in lib
is probably bad.
2. 'availability' and 'condition' are the two names used in code
to refer to this system ($CFG->enableavailability).
3. The prefix has to be short enough to allow database tables
(although in practice I assume that condition plugins will not
normally contain database tables).
The new API includes a Boolean tree structure that controls the
availability of an item.
AMOS BEGIN
CPY [availabilityconditions,core_condition],[restrictaccess,core_availability]
CPY [enableavailability,core_condition],[enableavailability,core_availability]
CPY [configenableavailability,core_condition],[enableavailability_desc,core_availability]
AMOS END
