R1:753828649956

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…