[storage] Classify cookies as site storage (#8)

Where did the "network storage" divide come from? I think of cookies as more like site storage since they are accessible to script. The http cache isn't AFAIK. Further, considering cookies as site storage gets us closer to the ideal of being able to pitch the "first-run" story to developers: if your origin gets purged due to quota pressure or `navigator.storage.purge()` you don't have to have any special logic to handle it, it will appear that the user has never visited your site before, so your first-run logic will suffice. Obviously, if we don't purge cookies then this story is a lie.

@slightlyoff said it well in his original durable storage page:
> what is an application without its Service Workers, Caches, Databases, Cookies, and all the rest? How is a developer meant to reason about having some but not all of these?

Of course we can't guarantee that the user hasn't modified any data locally, e.g. they could have deleted the IDB directory from the browser's profile directory and left the cookies, but at that point the user is on their own.

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https://github.com/whatwg/storage/issues/8