R1:9a32a563f047

Add a "View Task" button to HTML mail from Maniphest

Summary:
See downstream <https://phabricator.wikimedia.org/T1050>. Some time ago, we added a "View Revision" button to Differential mail. This hasn't created any problems and generally seems good / desirable.

It isn't trivial to just add everywhere since we need a translation string in each case, but at least add it to Maniphest for now. Going forward, we can fill in more applications as they come up.

Test Plan:
Used `bin/mail show-outbound --id <x>…