Lock down MetaMTA functionality to administrators

Summary:
We have a debug interface for sending various sorts of email, but normal users
don't really need to use it. In particular, they can:

• Send arbitrary email to other users;
• Discover other users' email addresses fairly easily (CC everyone);
• Send arbitrary email to arbitrary addresses in conjunction with "Mailing

Lists"

In fact, normal users don't need to get to the MetaMTA web interface at all and
it has some somewhat-sensitive things beacuse it has a lot of detailed
information about mail. For instance, users can look at mail records to discover
things like password reset links and per-user object email addresses.

We should smooth out the UI here but I think I can do something about T21 fairly
soon and cover it then.

Test Plan:
Went to /mail/ with a non-admin, got 404'd. Went to /mail/ with an
admin, everything works, got a red admin header.

Reviewers: jungejason, btrahan

Reviewed By: btrahan

CC: aran, btrahan, jungejason

Maniphest Tasks: T718

Differential Revision: https://secure.phabricator.com/D1292