Make named lookup of users case insensitive
Summary: Fixes T5302. Allow the name @aLiNCoLN to identify user @alincoln.
Test Plan: Queried users with mixed case names.
Reviewers: btrahan, spicyj, chad
Reviewed By: spicyj
Subscribers: epriestley
Maniphest Tasks: T5302
Differential Revision: https://secure.phabricator.com/D9451