Improve time localization code

Summary:
  - We throw on a missing date right now, in the DateTime constructor. This can
happen in reasonable cases and this is display code, so handle it more
gracefully (see T520).
  - This stuff is a little slow and we sometimes render many hundreds of dates
per page. I've been seeing it in profiles on and off. Memoize timezones to
improve performance.
  - Some minor code duplication that would have become less-minor with the
constructor change, consolidate the logic.
  - Add some unit tests and a little documentation.

Test Plan:
  - Ran unit tests.
  - Profiled 1,000 calls to phabricator_datetime(), cost dropped from ~49ms to
~19ms with addition of memoization. This is still slower than I'd like but I
don't think there's an easy way to squeeze it down further.

Reviewers: ajtrichards, jungejason, nh, tuomaspelkonen, aran

Reviewed By: ajtrichards

CC: aran, ajtrichards, epriestley

Differential Revision: 966

src/__phutil_library_map__.php

@@ -457,6 +457,7 @@ phutil_register_library_map(array(
     'PhabricatorLintEngine' => 'infrastructure/lint/engine',
     'PhabricatorLiskDAO' => 'applications/base/storage/lisk',
     'PhabricatorLocalDiskFileStorageEngine' => 'applications/files/engine/localdisk',
+    'PhabricatorLocalTimeTestCase' => 'view/utils/__tests__',
     'PhabricatorLoginController' => 'applications/auth/controller/login',
     'PhabricatorLogoutController' => 'applications/auth/controller/logout',
     'PhabricatorMailImplementationAdapter' => 'applications/metamta/adapter/base',
@@ -700,6 +701,7 @@ phutil_register_library_map(array(
   ),
   'function' =>
   array(
+    '__phabricator_format_local_time' => 'view/utils',
     '_qsprintf_check_scalar_type' => 'storage/qsprintf',
     '_qsprintf_check_type' => 'storage/qsprintf',
     'celerity_generate_unique_node_id' => 'infrastructure/celerity/api',
@@ -1073,6 +1075,7 @@ phutil_register_library_map(array(
     'PhabricatorLintEngine' => 'PhutilLintEngine',
     'PhabricatorLiskDAO' => 'LiskDAO',
     'PhabricatorLocalDiskFileStorageEngine' => 'PhabricatorFileStorageEngine',
+    'PhabricatorLocalTimeTestCase' => 'PhabricatorTestCase',
     'PhabricatorLoginController' => 'PhabricatorAuthController',
     'PhabricatorLogoutController' => 'PhabricatorAuthController',
     'PhabricatorMailImplementationAmazonSESAdapter' => 'PhabricatorMailImplementationPHPMailerLiteAdapter',