Add a "View Task" button to HTML mail from Maniphest
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.
Used bin/mail show-outbound --id <x> --dump-html:
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20561