Add javelin_tag(), convert easy callsites

Summary:
  - Implements `javelin_tag()`, which is `javelin_render_tag()` on top of `phutil_tag()` instead of `phutil_render_tag()`.
  - Manually converts all or almost all of the trivial callsites.

Test Plan:
  - Site does not seem any more broken than before.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4639

src/applications/countdown/controller/PhabricatorCountdownListController.php

@@ -36,7 +36,7 @@ final class PhabricatorCountdownListController
           ),
           'Edit');
 
-        $delete_button = javelin_render_tag(
+        $delete_button = javelin_tag(
           'a',
           array(
             'class' => 'small button grey',