Link to attached objects in email when a Maniphest task has stuff attached

Summary:
When files or revisions are attached to a Maniphest task, link to the new stuff
in the email. See T116.

Test Plan:
Attached files and revisions to a task, got sensible-looking emails about it.

Reviewed By: tomo
Reviewers: tomo, jungejason, tuomaspelkonen, aran
CC: aran, tomo
Differential Revision: 286