Make auth.email-domains case-insensitive

Summary:
Fixes T5143. Currently, if your allowed domain is "example.com", we reject signups from "@Example.com".

Instead, lowercase both parts before performing the check.

Test Plan:
  - Before patch:
    - Set allowed domains to "yghe.net".
    - Tried "x@yghe.net", no error.
    - Tried "x@xxxy.net", error.
    - Tried "x@yghE.net", incorrectly results in an error.
  - After patch:
    - Set allowed domains to "yghe.net".
    - Tried "x@yghe.net", no error.
    - Tried "x@xxxy.net", error.
    - Tried "x@yghE.net", this correctly no longer produces an error.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5143

Differential Revision: https://secure.phabricator.com/D9261