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> --dump-html:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20561