Save draft forms in local storage

[Floppy]

When a user is filling in a form and heads off to create a new collection or creator, the form content will be lost when they return. Ideally all forms should refill themselves with unsubmitted draft data if there is any.

Presumably this is possible using FormData and serializing into localStorage, but my initial experiments with FormData didn't work, so I'll come back to it later.

For someone who knows what they're doing with these things, it might be quite a quick job.