Support "Remix" for projects

[dethe]

Right now anyone can start with any project and modify it (or not) and save it under their own projects, which are all anonymous. What we should do now that we're adding the ability to log into github for saved gists, and beginning to support the MakeAPI, is have attribution fields (who wrote this) and a remix button to go into edit mode, so we can have attribution chains ('fred wrote this, remixed from a project by ethel'). Both Scratch and the MakeAPI do this, and it is worth supporting for Waterbear.