Add 'Report Issue' and Github icon

For the published viewer, it would be great if there was a link for users to click to log an issue. It should link to:

https://github.com/playcanvas/playcanvas-viewer/issues

Also, a Github logo (Octocat) that links to the top level of the repo would be awesome too (to make it clear this is an open source project that devs can contribute to):

https://github.com/playcanvas/playcanvas-viewer