phutil_utf8_shorten => PhutilUTF8StringTruncator

Summary: Fixes T3307. Clean up the remaining instances and delete the actual function

Test Plan: arc lintypoo

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3307

Differential Revision: https://secure.phabricator.com/D10390