Resurrect setup check for cluster.mailers

Summary:
D19940 removed this file entirely, which has led to at least one user who was unsure how to proceed now that cluster.mailers is required for outbound mail: https://discourse.phabricator-community.org/t/invalid-argument-supplied-for-foreach-phabricatormetamtamail-php/2287

This isn't always a setup issue for installs that don't care about sending mail, but this at least this gives a sporting chance to users who don't follow the changelogs.

Also, I'm not sure if there's a way to use pht() to generate links; right now the phurl is just in plain text.

Test Plan: Removed cluster.mailers config; observed expected setup issue.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19964