Position the uploader window into the content section of the screen.

The admin menu needs to be able to overlap the content area for
flyout menus, but we want the uploader window takeover to cover
the entire screen. Without refactoring markup, this can serve
as a stop gap for now.
Fix styles for bulk editing buttons `clear` and `delete` buttons.

Clear is not a neutral button (blue/white colors) delete is now a
warning button (red/white colors).
While bulk editing, make any selected attachment styled the same as other selected attachments.