Make bulk editor working set editable and more homogenous

Summary:
Ref T13025. See PHI50. Fixes T11286. Ref T10005. Begin modernizing the bulk editor.

For T10005 ("move the bulk editor to modern infrastructure"), rewrite the rendering of the editable set so that it is application-agnostic and can work with any kind of object.

For T11286 ("let users de-select items in the working set"), make the working set editable.

Test Plan:
{F5302158}

• Deselected some objects, applied an edit, saw the edit apply to only selected objects.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025, T11286, T10005

Differential Revision: https://secure.phabricator.com/D18805