Save, load proofs

[rzach]

Add a way to download a proof in some format and to upload proofs in the same format to an existing proof table; and/or ability to save a proof (maybe in a cookie?)

[benburrill]

If this was done through query parameters (or fragment), it would also allow the creation of urls that link directly to a (possibly incomplete) proof for use as an exercise. These urls would also work as saves.