Store proofs in local storage

[io12]

(So people don't accidentally delete proofs on refresh or closing the tab)

[aweinstock314]

Reposting from slack:

It looks like 
https://docs.rs/web-sys/0.3.36/web_sys/struct.Window.html#method.local_storage 
would be a good way to get access to the local storage object. The end of
ProofWidget::update would be a good place to save the xml for the current proof
(if ret is true, a modification has just occurred). For loading from local
storage, a few intertwined things in App::{update,view} would need to change.