R1:a76f61f7e183

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.