R1:7cf0358dda11

Disallow email addresses which will overflow MySQL storage

Summary:
Via HackerOne. An attacker can bypass `auth.email-domains` by registering with an email like:

aaaaa...aaaaa@evil.com@company.com

We'll validate the full string, then insert it into the database where it will be truncated, removing the `@company.com` part. Then we'll send an email to `@evil.com`.

Instead, reject email addresses which won't fit in the table.

`STRICT_ALL_TABLES` stops this attack, I'm going to add a setup warning…