Move mail-related setup issues to setup checks

Summary: Ports mail stuff from the existing setup process to the more modular setup checks.

Test Plan: Configured my local install to have all these errors, verified setup raised them.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4494