Replace the unit page with the container page

STUD-1754
I was naively posting the update based upon other methods we have, but clearly this should be going through the model as you pointed out. I created a helper method but it got very small once I refactored it to use ```runOperationShowingMessage```, so I just inlined the two versions. I think I prefer this anyway because XBlockInfo isn't a view so doesn't have access to that method, but putting it in xblock.js had other issues.