Add capabilities for editing task triage details (priority, assignee, etc)

Summary:
This is primarily a client request, and a little bit use-case specific, but policies seem to be holding up well and I'm getting more comfortable about maintaining this. Much if it can run through ApplicationTransactions.

Allow the ability to edit status, policies, priorities, assignees and projects of a task to be restricted to some subset of users. Also allow bulk edit to be locked. This affects the editor itself and the edit, view and list interfaces.

Test Plan: As a restricted user, created, edited and commented on tasks. Tried to drag them around.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7357