phutil_utf8_shorten => PhutilUTF8StringTruncator

Summary: Ref T3307. Only one I thought was tricky was Excel; I went with bytes there like it was email.

Test Plan: played around on a few endpoints but mostly thought carefully

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3307

Differential Revision: https://secure.phabricator.com/D10392