Match usernames in any case in commit message object lists (CC, Reviewers, etc.)

Summary:
Allow entry of "CC: alincoln" to match user "ALincoln".

Put both variations in the map and try the exact case version first since we'll
also match email addresses and mailables, and theoretically some mailable might
have the same name as a user, as we're effectively abandoning restriction of
which characters can appear in usernames.

Test Plan: Created a local revision with a reviewer in CrAzY CaPs.

Reviewers: jungejason, btrahan

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T697

Differential Revision: 1255