Fix #17751: add 'UTC' to timezone selection list

Also fixes the list for timezones containing a '/' in the name (e.g.
Argentina/xxx)
Fix timezone initialization in core.php

Prior to this, core.php would trigger a PHP warning on PHP >= 5.3 when
date_default_timezone_get() is not able to determine the timezone and
defaults to UTC (e.g. when php.ini date.timezone is not set).

The timezone is determined in the following order
  1. user preferences
  2. MantisBT configuration ($g_default_timezone)
  3. php.ini setting (date.timezone)
  4. UTC (set by date_default_timezone_get())

Fixes #17747
Revised admin check for $g_default_timezone

- Clarification of messages, added links to PHP documentation
- When default timezone is null, issue warning instead of failure and
  indicate defaulting to UTC
Update documentation for $g_default_timezone
Additional timezone init fixes

Implements additional fixes submitted by grangeway in PR #387

- Set installer_db_now() timezone to UTC
- Remove timezone_identifiers_list() existence checks
  These checks are obsolete, there were implemented to cover RHEL/CentOS
  under PHP 5.1 and we now require 5.3.
- Travis: set timezone to UTC

Signed-off-by: Damien Regad <dregad@mantisbt.org>
Set default timezone in install.php
Improve Timezone initialization, default to UTC

Fixes https://github.com/mantisbt/mantisbt/pull/382