Fix duplicate save and cancel buttons showing when editing xblocks

These changes fix the issue reported as STUD-1531. The basic problem was that the new edit modal code was not updated to understand @wedaly's changes to allow xblocks to provide their own buttons. This change allows any xblock to add the class "xblock-with-editor" to itself, which will cause Studio to hide its own buttons. In addition, the CMS xblock runtime has been updated to use the newer mechanism of closing the edit modal.

Finally, some legacy logic has been removed, and unit tests have been updated to reflect this.