Initial commit. New media workflow `manage`, based on select for now.
Image editing
Render content region appropriately for edit image state in Manage flow
Responsive parity with admin menu
Add small, medium, and large sizing options.

For now, this "works," but I'd like to revisit to review all the
use cases that Attachments views or Library controllers have a
custom `edge` set, and whether or not we should backwards compat
support them all, or make them flex as well.
Remove the toolbar within the Manage media frame.
Add a bulk edit mode.

Add an option to wp.media.view.AttachmentsBrowser `bulkEdit`,
which defaults to false. When set to true, conditionally create
a SelectionBulkEdit view when multiple items are selected.

Multiple delete now possible.
Fix batch deleting only deleting half of the models.