Fix Maniphest and Config transaction diff rendering

Summary:
I missed this in review of D5155: `wordwrap()` returns a string, but `phutil_utf8_hard_wrap()` returns an array.

Implode the returned arrays so the stuff underneath it doesn't choke.

Test Plan: Clicked "show details" on a maniphest task description change

Reviewers: AnhNhan, kwadwon

Reviewed By: kwadwon

CC: aran

Differential Revision: https://secure.phabricator.com/D5195