Maniphest - add 'upforgrabs' hint in bulk editor

Summary: change typeahead placeholder to include 'or "upforgrabs" to unassign...' Fixes T2267. Well, makes it as good as its going to be until we get some new space age UI.

Test Plan: new text looks okay-ish

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T2267

Differential Revision: https://secure.phabricator.com/D9882