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