Resurrect setup check for cluster.mailers
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.
Reviewed By: epriestley
Differential Revision: https://secure.phabricator.com/D19964