Improve setup process / documentation for outbound email configuration

Summary:
ccheever did an install and gave me some feedback about issues he hit. This
tries to:

• properly document how to configure outbound email;
• test outbound email configuration in the setup mode;
• provide basic daemon documentation;
• document that phabricator.base-uri is required for all installs.

Test Plan:
read documentation, jumped through all the setup branches to test configuration
error detection

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran, rm
CC: ccheever, aran
Differential Revision: 276