Replace custom Moodle timezone stuff with standard PHP date/time code

Moodle timezone stuff is buggy, unreliable and hard to use. Moreover it is incompatible with standard PHP date functions and classes.

Pretty much all places that use $CFG->timezone and $user->timezone directly (outside of admin settings and user profiles) are bugs at the moment, this patch introduces new core_date::get_system_timezone() and core_date::get_user_timezone($user = null) which return timezone name compatible both with Moodle legacy date functions and standard PHP code.

Backwards compatibility should be excellent both for data and existing code.

Remaining issues that are not solved by this patch:

• the badzone list could be expanded to cover all those strange Windows timezones and maybe some more abbreviations
• some parts of Moodle need to be properly cleaned up/rewritten to use PHP time intervals - stats, calendar, etc.
• 3rd party calendars need to be updated
• there should be some simple docs for developers
• there could be an environment check for outdated PHP timezones - if older than 1 year for example

Required documentation:

• developers - they need to understand the new core_date class and they need to use it always
• administrators should know that the built-in timezone data can be updated via pecl extension - see http://pecl.php.net/package/timezonedb