Replace array_interleave() by phutil_implode_html()

Summary:
I like this abstraction better.
Result of `phutil_implode_html()` may be also used as a param of `hsprintf()`.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4904

src/applications/help/controller/PhabricatorHelpKeyboardShortcutController.php

@@ -27,7 +27,7 @@ final class PhabricatorHelpKeyboardShortcutController
       foreach ($shortcut['keys'] as $stroke) {
         $keystrokes[] = phutil_tag('kbd', array(), $stroke);
       }
-      $keystrokes = array_interleave(' or ', $keystrokes);
+      $keystrokes = phutil_implode_html(' or ', $keystrokes);
       $rows[] = phutil_tag(
         'tr',
         array(),