update Skulpt and OnlinePythonTutor
handle play button overlay on different sized thumbnails a little better
give a friendly error message when someone tries to create a course with a duplicate name
Merge pull request #229 from isaacdontjelindell/submodule-issue-219

update Skulpt and OnlinePythonTutor
Merge pull request #231 from isaacdontjelindell/video-overlay

Second try at play button video overlay
Merge pull request #232 from isaacdontjelindell/issue-230

Give a friendly error message when someone tries to create a course with a duplicate name