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