MDL-49684 timezones: rewrite timezone support

MDL-49684 timezones: rewrite timezone support

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