When sorting subscriber references for display in the curtain UI, sort without case sensitivity

Summary: Ref T13486. Currently, "Zarbo" sorts above "alice", but this isn't expected for a list of (mostly) human usernames.

Test Plan: Loaded a task with subscribers with mixed-case usernames.

Maniphest Tasks: T13486

Differential Revision: https://secure.phabricator.com/D20969