Add "View Email" action to application transactions

Summary: Should help with debugging. In the web UI there is now a link to "View Email". This uses the same debugging output that `./bin/mail show-outbound` uses. Fixes T5768. Code is very defensive as I think these tables may be truncated eventually?

Test Plan: viewed some emails and it worked!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5768

Differential Revision: https://secure.phabricator.com/D10244