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 encouraging it.

Test Plan:
  - Set `auth.email-domains` to `@company.com`.
  - Registered with `aaa...aaa@evil.com@company.com`. Previously this worked, now it is rejected.
  - Did a valid registration.
  - Tried to add `aaa...aaaa@evil.com@company.com` as an email address. Previously this worked, now it is rejected.
  - Did a valid email add.
  - Added and executed unit tests.

Reviewers: btrahan, arice

Reviewed By: arice

CC: aran, chad

Differential Revision: https://secure.phabricator.com/D8308