Iframe reload optimizations for various xblock related actions. Added some improvements related to scrolling to the current xblock. Fixed behavior of the xblock action dropdown list.
This is part of the effort to support the new embedded Studio Unit Page. It includes changes to the CourseUnit component and the functionality of interaction between xblocks in the iframe and the react page.
The following events have been processed:
* delete event
* Manage Access event (opening and closing a modal window)
* edit event for new xblock React editors
* clipboard events
* duplicate event
This refactors the CourseUnit component by removing the DraggableList and CourseXBlock components and replacing them with a simpler XBlockContainerIframe. Additionally, it introduces new constants for iframe handling.
