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