Render timezones in event reminder mail, and render them more nicely

Summary:
Fixes T12356.

• In this mail, we currently render "6:00 AM". Instead, render "6:00 AM (PDT)" or similar. This is consistent with times in other modern Transaction mail.
• Previously, we would render "UTC-7". Render "PDT" instead. For obscure zones with no known timezone abbreviation, fall back to "UTC-7".

Test Plan:

• Used bin/calendar notify --minutes X to trigger notifications, read email bodies.
• Used this script to list all T values and checked them for sanity:

<?php

$now = new DateTime();

$locales = DateTimeZone::listIdentifiers();
foreach ($locales as $locale) {
  $zone = new DateTimeZone($locale);
  $now->setTimeZone($zone);

  printf(
    "%s (%s)\n",
    $locale,
    $now->format('T'));
}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12356

Differential Revision: https://secure.phabricator.com/D17646