Provide helper function for <div class="%s">%s</div>

Summary: I don't like helper functions but this is used often and it makes the code more readable.

Test Plan: Will use it.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7545