Add diff view for Maniphest Task "description changed" transactions

Summary:
use the handy DifferentialChangesetParser to do most of the heavy lifting inside
the pertinent view object. update the controller to be aware of the "show
more" calls coming from the new ui and update the transactionID appropriately.

also snuck in a small change to AprontRequest to all getting all the request
data. I used it to debug building this.

Test Plan: made a task and entered a bunch of test data. had descriptions of
various lengths, as well as really long descriptions that i did not change to
much. verified the diff looked correct and various "show more" links worked as
expected

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Differential Revision: 1187