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