Encode "<" and ">" in JSON/Ajax responses to prevent content-sniffing attacks

Summary:
Some browsers will still sniff content types even with "Content-Type" and
"X-Content-Type-Options: nosniff". Encode "<" and ">" to prevent them from
sniffing the content as HTML.

See T865.

Also unified some of the code on this pathway.

Test Plan: Verified Opera no longer sniffs the Conduit response into HTML for
the test case in T865. Unit tests pass.

Reviewers: cbg, btrahan

Reviewed By: cbg

CC: aran, epriestley

Maniphest Tasks: T139, T865

Differential Revision: https://secure.phabricator.com/D1606

src/__phutil_library_map__.php

@@ -55,6 +55,7 @@ phutil_register_library_map(array(
     'AphrontIsolatedDatabaseConnection' => 'storage/connection/isolated',
     'AphrontIsolatedDatabaseConnectionTestCase' => 'storage/connection/isolated/__tests__',
     'AphrontIsolatedHTTPSink' => 'aphront/sink/test',
+    'AphrontJSONResponse' => 'aphront/response/json',
     'AphrontJavelinView' => 'view/javelin-view',
     'AphrontKeyboardShortcutsAvailableView' => 'view/widget/keyboardshortcuts',
     'AphrontListFilterView' => 'view/layout/listfilter',
@@ -880,6 +881,7 @@ phutil_register_library_map(array(
     'AphrontIsolatedDatabaseConnection' => 'AphrontDatabaseConnection',
     'AphrontIsolatedDatabaseConnectionTestCase' => 'PhabricatorTestCase',
     'AphrontIsolatedHTTPSink' => 'AphrontHTTPSink',
+    'AphrontJSONResponse' => 'AphrontResponse',
     'AphrontJavelinView' => 'AphrontView',
     'AphrontKeyboardShortcutsAvailableView' => 'AphrontView',
     'AphrontListFilterView' => 'AphrontView',