Add "Edit" button to each leaf xblock on the container page

This is the set of changes to implement STUD-1306. The current xblock edit modal on the unit page has been replaced with a newly designed modal. The new one is a true modal that appears in the middle of the window, and it can be opened from any page (the existing one only works if an xblock has been rendered directly onto the page). The primary reason for the new modal is to support editing from the container page, where components can be nested and are not rendered as individual xblocks.

Note: the old modal has been replaced on the unit and "Pages" page too.

You can view these changes on my sandbox using "Test Course":

http://studio.andy-armstrong.m.sandbox.edx.org/settings/advanced/AndyA.AA101.AndyA/branch/draft/block/AndyA