Tweak MetaMTA test console a bit for usability

Summary: default check the system send prefernce for immediateness and add more direct text about dameons, with a link to help.

Test Plan: looks good

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T726

Differential Revision: https://secure.phabricator.com/D3262