LocalStorage is never cleared

[Zensho]

Currently when running in static file mode, the program saves a lot of information in LocalStorage, but never makes any attempt to clear it. This results in a bloated LocalStorage when using the same url and affects opening/executing speed of the program.

I think this might be because the undo-manager is creating a lot of information, and also because the getTreebankId() method which is supposed to access and clean up LocalStorage entries is not working as intended, leading to the program not keeping the corpus on page refresh (#339) and also not clearing the LocalStorage at all.