Rough batch editor for Maniphest

Summary:
First stab at a batch editor for Maniphest. Basically, you can select a group of
tasks and then import them into the "batch" interface, where you can edit all of
them at once.

High level goal is to make it easier for users in PM/filer/support/QA roles to
deal with large numbers of tasks quickly.

This implementation has a few major limitations:

• The only available actions are "add projects" and "remove projects".
• There is no review / undo / log stuff.
• All the changes are applied in-process, which may not scale terribly well.

However, the immediate need is just around projects and this seemed like a
reasonable place to draw the line for a minimal useful version of the tool.

Test Plan: Used batch editor to add and remove projects from groups of tasks.

Reviewers: btrahan, yairlivne

Reviewed By: btrahan

CC: aran, epriestley, sandra

Maniphest Tasks: T441

Differential Revision: https://secure.phabricator.com/D1680