Playlab gallery

* capture screenshots of playlab solutions for sharing. Currently all this does is capture the state of the play area when the "Finish" button is pressed. (future options for refinement could include using the title screen text, the state of the play area after the "when run" blocks are finished, or giving the user multiple options for screenshots) This is done via canvg, which serializes and parses the <svg> element and renders it on canvas, then converts the canvas to png. See:
  * https://code.google.com/p/canvg/
  * https://github.com/sampumon/SVG.toDataURL
The libraries for this are all in a /canvg directory under /src, is this the right place to put these?
* I had to increase the maximum size of a POST request accepted by the server because these data url pngs are pretty large. (the size of the db column that holds these images is 16M, we should be fine there)
* show both "apps" and "art" on the main public gallery page
* do not show autosaved drawings in the public gallery
* add some constants to Game so we can use Game::ARTIST instead of 'turtle' ;)

TODO:
* ui test (can't really test the contents of the screenshot automatically, but can test that it doesn't break the Finish button)
* include the css for the svg elements in the screenshot (title screen among other things looks weird)
* split the personal gallery into "My Apps" and "My Art"
* pagination for private gallery
* classroom gallery