Allow editing of container xblocks/xmodules

These changes implement STUD-1312, to allow editing of container xblocks/xmodules. There are three ways in which a container can be edited:
 - from an "Edit" button shown when a container is a child of a unit page
 - from an "Edit" button shown when a container is a child of a container page
 - from the main "Edit" button at the top right of the container page itself