Add option to make storage persistent to Offline page.

[pwnall]

Fixes #703

I tested this manually and it works for me in Chrome.

I looked around a bit and focused on getting something working. I probably got the architecture bits wrong -- I'd be happy to update my PR in response to architectural feedback.

I hope this helps!

[drizzle042]

Fixes #703

I tested this manually and it works for me in Chrome.

I looked around a bit and focused on getting something working. I probably got the architecture bits wrong -- I'd be happy to update my PR in response to architectural feedback.

I hope this helps!

What do you think of also adding sign up feature to enable user saved data persist even when caches are cleared Eg enabled docs staying enabled