Fix missing link targets for "View Object" header buttons in HTML email

Summary:
See https://discourse.phabricator-community.org/t/view-task-from-maniphest-e-mail-doesnt-have-url/2827.

I added "View Task" / "View Commit" buttons recently but the logic for generating URIs isn't quite right. Fix it up.

Test Plan:

• Commented on a task.
• Used bin/mail show-outbound --id ... --dump-html > out.html to dump the HTML.
• Previewed the HTML in a browser.
• This time, actually clicked the button to go to the task.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20586