"Merge Duplicates" in Maniphest

Summary:
Allow duplicate tasks to be selected and merged in Maniphest.

I didn't create a separate transaction type for this because that implies a
bunch of really complicated rules which I don't want to sort out right now
(e.g., do we need to do cycle detection for merges? If so, what do we do when we
detect a cycle?) since I think it's unnecessary to get right for the initial
implementation (my Tasks merge implementation was similar to this and worked
quite well) and if/when we eventually need the metadata to be available in a
computer-readable form that need should inform the implementation.

Plenty of room for improvement here, of course.

Test Plan:
Merged duplicate tasks, tried to perform invalid merge operations (e.g., merge a
task into itself).
Tested existing attach workflows (task -> revision, revision -> task).

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: anjali, aran
Differential Revision: 459