This patch replaces all homegrown timezone
stuff with standard PHP date/time code.

The main change is the introduction of core_date
class that returns normalised user and server
timezones. From now on nobody should be using
$CFG->timezone or $user->timezone directly!

Other new features and fixes:
* admins are prompted for timezone during install
* editing of other users is finally fixed
* timezones are displayed in user profile
* new $this->setTimezone() in phpunit
* time locale is now automatically reset in phpunit
* timezone is now automatically reset in phpunit
* phpunit has Australia/Perth as default timezone

Includes all the conditions that were in previous Moodle versions:

* Date
* Grade
* Completion (of another activity)
* User profile field

Also includes conditions that are used to reimplement
groupmembersonly:

* Grouping
* Group

For each condition, the component plus unit tests are included.

PLEASE NOTE: The code to actually check each condition is reused
from previous Moodle versions and has not been modified except to
pass codechecker. This is intentional, to reduce the risk of the
change and maximise the chance that behaviour is preserved. Some
of this code might not be very good and might need updating but
that can happen separately.

AMOS BEGIN
 CPY [contains,core_condition],[op_contains,availability_profile]
 CPY [doesnotcontain,core_condition],[op_doesnotcontain,availability_profile]
 CPY [endswith,core_condition],[op_endswith,availability_profile]
 CPY [isempty,core_condition],[op_isempty,availability_profile]
 CPY [isequalto,core_condition],[op_isequalto,availability_profile]
 CPY [isnotempty,core_condition],[op_isnotempty,availability_profile]
 CPY [startswith,core_condition],[op_startswith,availability_profile]
 CPY [completion_fail,core_condition],[option_fail,availability_completion]
 CPY [completion_pass,core_condition],[option_pass,availability_completion]
 CPY [completion_complete,core_condition],[option_complete,availability_completion]
 CPY [completion_incomplete,core_condition],[option_incomplete,availability_completion]
AMOS END