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