Add a "View Object" button on the web mail view page

Summary:
Depends on D19015. Ref T13053. Currently, we don't link up hyperlinks in the body of mail viewed in the web UI. We should, but this is a little tricky (see T13053#235074).

As a general improvement to make working with "Must Encrypt" mail less painful, add a big button to jump to the related object.

Test Plan: {F5415990}

Reviewers: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19016