give each app its own page, preloading and session stores, using context

[Rich-Harris]

This is an alternative to #582.

To recap: we need a way to access page, preloading and session inside an app — as stores — that's consistent between server and client. One way to do that is to export them from @sapper/app directly, and rely on the fact that we can reset them immediately before server rendering to ensure that session data isn't accidentally leaked between two users accessing the same server.

It's not future-proof, though. If at some point we support asynchronous or streaming rendering, it would be possible for an app to write to the session store in a way that could affect its value for a different user, if the other user read session asynchronously.

The only solution that I can see is to ensure that each user gets their own set of stores for each server-rendered page. We can achieve this with the context API, and expose the stores like so:

<script>
  import { stores } from '@sapper/app';
  const { page, preloading, session } = stores();
</script>

Calling stores() outside component initialisation would be an error.