Show description changes in Maniphest

Summary:
When a task description is updated, there's currently no way to see the change.
Build an "expanded summary" mode for transactions that shows description change
details. Also include changes in the email.

Test Plan:
Changed task descriptions, clicked "show details", read email.

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: anjali, aran, epriestley
Differential Revision: 320